I never knew about the nail polish shelf. That is a cool thing and I will have to remember it. I know how the local game stores have their loose cart displays, in some wooden arrangement with a plexiglass cover. It wouldn't be too complicated to build if you could do it yourself. I do not have a huge amount of game boy games, but if that nail polish thing works for Game Gear I may try it out.
Currently, I keep all my loose portable cart games in a Jeff Gordon pencil case.
There is a YouTube video on that, any cartridge based games should work for it. As pong as they aren't to tiny like the gba.