Right idea, yea. This should work fine if you make the shoulder buttons using tact switches. I remember the GC controller having a lot of travel on the shoulder buttons (I believe they were analog but not 100% on that, it was definitely useful in ssbm). If you want to use the actual button connections from the controller, you will have to cut into the side of the case to all your index fingers to travel the length of the button travel.
Looks to be a comfortably sized portable and looks really sleek as it is. Good luck on the process and keep us updated. I'm just learning a lot of this stuff myself and look forward to anything I could learn from you as well helping any way that I can.
I thought the shoulder buttons on a gc controller were pressure sensitive?
Also, if that is the on/off switch on the top right of the case, by the R button, I'd make sure the Left side of the switch is the on position so you don't accidentally push the system off during a bit of excited gameplay
(Unless you have oversized index fingers, I can't imagine easily sliding/pulling the switch right without actually trying to)
Ah yes good idea