Will the NES Game Genie fit in your NES cases?
Unfortunately, no, NES Game Genies won't fit in my NES Game Case, as it's about 3/4 of an inch too tall...
Well, if you take the black plastic strap off, it'll fit.
Say, when you connect two NES Game Genies together,
or connect a NES Game Genie to the Top-Loader Adapter,
how does their height compare to a NES Game (not counting strap length)?
Don't toss that plastic strap, though, because later on, when I start making the one-piece, extra tall
DVD Style Cases, for Saturn CD Games and Carts, a NES Game Genie will fit in there, with the strap.
I bought some from somewhere else and the weren't much better. They were black and the plastic was very brittle and cracked when you looked at it.
These are far better quality 
http://www.genesysdtp.com/index.htm?nintendodscasedetails.htm
Those were the cases Seymor linked at the end of the previous page. Evidently they dont have the gba slot anymore.
There have been reports that the plastic bump in the case has to be cut out, to get GBA Games in em'.

Even then, I'm not sure if the GBA Area has grips on it's sidewalls, or not.
I can't tell from the picture.