There are no good options for game gear games. Just putting them on the shelf makes it impossible to know what you have you can't even write on the edge or put a side label. Case solutions I found online are basically either find DS/3ds cases and hack them all up, but these are harder to source, more expensive both for the base and the printing and they don't look the best, or you can go the customgamecases route which is insanely expensive if you have a large or complete collection. I've gone with a different option.
These are some custom cases I've made for my game gear games. These are basically my mk.1 design. There are some slight changes/additions/modifications I'm going to make to both the covers I designed and the cases themselves. I'm using standard cassette cases because they're cheap and available. The games are just a hair too big so I need to cut a hole in the side of the cases. I seen a post from Reddit where someone did a case without cutting and they either can't easily open the case or found slightly larger than average cases. I'm hoping to clean up my cutting so it at least looks a little cleaner. The place where I sanded off the post looks bad so I'm going to maybe slap a sticker over it, just haven't decided what.
I'm not saying this is the best or even a good option, I'm saying all the options suck and I'm just showing off my sucky choice.
Those are all interesting ideas but just like mine there's always some kind of catch. either cost, space or appeal. The final one you showed looks great but it is the problem of cost/time to make. I guess this is the trade-off for all of the games being dirt cheap basically.
Upon second viewing, I highly suspect he glued that cover onto the case... which will warp/peel off SO FAST.
There is a reason cases have a plastic shield and lips on the case to hold the cover in.