Styrene, styrene, and more styrene! Seriously though, I've done a fair amount of scale modeling, and everything I picked up doing that has helped a lot for making cases and other such things. Anyways, lets get things rolling here.
You're going to need a fair amount of tools and materials to make a decent looking case out of sheet plastic. I have access to a large variety of things (band saw, drill press, etc) that a lot of people probably don't have, but all that means is it might take a bit longer to cut things. Aside from larger cutting tools and a drill of some sort, you're going to need:
-an x-acto/utility knife
-sharp scissors or plastic snips
-a heat gun
-superglue (cyano acrylate, 15-20 second curing works best)
-talcum baby powder (more on this later) or polyester putty
-plastic cement (I use the liquid stuff from Testors, it works much better than the gel)
-sand paper (200 grit for rough sanding, 800+ for smoothing)
-plastic (of course)
Pretty much all of those materials can be found at a hobby store. For superglue, look for larger bottles (1oz. +) as they are generally much more cost effective than getting little tubes of crazy glue. And if you're having trouble finding styrene, look for hobby stores that sell model train supplies. Oh, and having a dremel tool with sanding bits will make things a lot easier.
Styrene is a fairly standard plastic for adding on to scale models. You can get it in a pretty large range of thicknesses, from paper thin to over 1/16". Needless to say, you're going to want something thick, but not too thick to work with. I like to use .06" to .08" thick for most things.
Cutting styrene in straight lines is very easy. You simply mark off where you want to cut, score it with a blade, then snap the piece off. Cutting curves is a little more difficult. You can do it by scoring, but its probably going to be a bit harder to draw a curve with a blade in the plastic. I generally cut strips, then use scissors or plastic snips (cutters) to get as close as possible to the shape that I want, then clean it up with a blade and sand paper.
Bending styrene is also pretty easy to do. It helps if you have something to hold the styrene down with (such as a strip of metal and tape, or a clamp), but all you really need is a heat gun of some sort. Hairdryers will work for bends in a small area, but they usually take too long to heat up the plastic for something large. I use an old embossing gun that we had lying around, but anything that will radiate a large amount of heat (or blow hot air) will work. You want to avoid using a flame of any kind though, as you are more likely to burn the plastic and release fumes.
As for glueing styrene, any plastic cement you buy is likely to have instructions on the packaging. Follow them and you should be fine.
Starting the case:
After you've planned everything out (sizes, placement of parts, etc) there are a couple of different ways to getting started. You can either get a pre-made box for a base (ie a project box), or start building from scratch. Regardless of which way I do it, I always like to build the case in two main parts: a front panel, and a "box" that will hold everything else. I generally make the front panel first, because if something doesn't work out as planned (a part sticks out too far), it is much easier to adapt the box to compensate for the required space than to rework the front panel.
The front panel (and holding things in place):
I like to mount everything practically possible on the front panel. This is usually the LCD, LCD controller board, and all of the controls (save shoulder buttons). In most cases, whatever screen you are using for your portable is going to have been mounted in its original case using screws. So, instead of glueing things in place, why not take adventage of the screw holes?
The easiest way I find to mount things with screws is using, you guessed it, styrene. Making a screw peg is pretty simple. Just cut a bunch of small squares of styrene and stack them together until you have the height you want. After the glue has had plenty of time to dry, drill a hole down the center about the same size as the diameter of the screw you want to use (the main part of the screw, not including the threads!). As long as you the screw you are using doesn't have very fine threads, it should thread the hole the first time you screw it in and hold tight once its in all the way.
For attaching the screw pegs, plastic cement should do the trick in most cases. A word of warning if you are using an ABS project box though. Although the plastic cement I have is supposed to bond normal styrene to ABS, it did not hold very well when I tried it on my second case. If you are trying to attach styrene to one of these ABS boxes, I found it worked best just to use the super glue. Just make sure that both surfaces are completely flat and clean, and it should hold without a problem. This also goes for any other occurance of glueing styrene to the ABS project boxes.
The rest of the case:
Now that you've got the front of the case done, everything else should be pretty straight forward. Use more screw pegs where ever necessary, and dont be afraid to permanently glue things in place (just make sure they're exactly where you want them to be), but try not to put these parts in until the case is finished and painted. If you need to any large holes in the middle of the case, mark what needs to be cut, and drill a hole in each of the corners to have room to work with scissors or plastic snips. For battery covers, having screw-on panels is the easiest way to go. You can try to make a snap-in cover, but it's not really worth the effort. And for attaching the front panel to the rest of the case? If you've used a project box, its as simple as screwing the top onto the rest of the box. If you've scratch built the case, you can either use more screw pegs, or if you're incredibly confident in your work, glue the two pieces together.
Cleaning things up:
Now that everything is assembled, you probably have a case with a lot of sharp corners and not-so-even looking edges. If you want to fill in those gaps or put a nice curve on inside corners, you can use polyester putty. However, if you want something that will cure fast, and be workable in under 10 minutes, then heres where that talcam powder comes into play.
Using some sort of mixing tray, (I like to use covers from cheap plastic food containers), pour in a fair amount of talcum powder. Then, on a different part of the tray, pour a good amount of superglue. Slowly mix the talcum powder (toothpicks work well for this) into the superglue until it starts to become a little stringy (like chewing gum). Mix it a bit more to try and get out any airbubbles, then apply it to whatever area it is you want to build up. It's going to be difficult to try and apply it in the shape you want, so you're better off putting down a glob then cutting/sanding it down after it dries.
Some words of warning for using this method. Cyano acrylate is bad for you (if you live in CA, it causes cancer!
) and you're going to want to have good ventilation when you do this. Second, you're using superglue here. This stuff is going to stick to just about everything, so be careful when you apply it. If something is so important that you absolutely can't mess it up, mask the area off. Also, when you sand it, use a dust mask! The dust particals created when sanding are still cyano acrylate, so they're still bad for you. And if you do use this method, experiment on scrap first to get a hang of how to do it. Practice getting a good ratio of superglue to talcam powder, and figure out how much time you have to work with it before it starts to harden. It's also important that you DO NOT use this for holding things together. It may seem like a strong bond, but after handling the parts for a while, they may come apart. Use plain superglue (or another adhesive) for attaching structural parts to each other.
Once everything is dry, you can get to work on sanding it however you want. I usually use a dremel tool to sand everything down to about where I want it to be, then clean it up with a utility knife and sandpaper.
You should try to sand everything as smooth as possible, because even if you can't see scratches or little dents as is, they will really stand out once you paint the case. Before you start painting, choose a paint that is going to work well. I haven't tried Krylon's plastic paint, but I can not recommend Rustoleum. I've tried it on a couple of different things and although it sticks to flat surfaces fine, it seems to rub off on corners. For any other paint, use a plastic primer (scale model enemal primer works well) then paint it with whatever you want. Remember of course to mask things off that you dont want to get paint on.
That's it for now, I'll post more if there's anything else I can think of.