First off, big thanks to lovablechevy for the idea and instructions.
I've wanted to make one of these for awhile, but was concerned it'd get stuck in my toploader NES. I had a Gyromite cart with the fabled converter inside, and I got the idea of using a Game Genie shell when I realized how close the fit was, and that the GG has a convenient pull-handle.
Here's a pic of the converter on top of an unmodified GG (the F-F slot on this converter was already flipped upside down, allowing for more clearance on top for the Fami cart to slide onto). This didn't look like it'd be hard at all.

After posting a request on DigitPress, a member over there came through with THREE empty GG shells for me to play with (which was a good thing, as I ended up ruining the first one).
So I went to work on the adapter itself. In these pics, the F-F slot has been ground down a bit, and the screw tabs have been cut (I left a little bit of them there for reference and to act as a 'stopper' while I worked on fitting it in the shell). Having the male adapter sunk completely into the F-F slot, the whole thing was just a bit too short to reach both the Fami cart and the cart slot on the NES, so I slid the two pieces apart just a tiny bit (2-3mm) and dumped on the hot glue.

Some test fitting. I wanted to make sure the cart would sit absolutely flush on top of the adapter, so leaving the cart plugged in (which is a tight fit) worked as an excellent guide for positioning everything.


Once I got the positioning correct, I was ready to seal up. Since I'd lost 2/4 of the stock screw holes (had to grind them down inside to make room for everything) I decided once again that the hot glue gun was going to be my savior. I would've never trusted this with just two screws holding it together, but now I could probably tow a car with it.
Final pics. I'll probably paint it eventually, but it's fine for now.




And a crappy pic of it installed. I couldn't get a decent pic of the NES in it's home, so I had to unplug it and pull it out. Yes, it works!

Thanks again to lovablechevy for the original idea. Now I can finally start collecting some Famicom titles!
(And yes, I know Tetris was available in the US also. I needed a cheap Famicom cart for fitting assistance.)