I have about 15 of each of these back button boards that use the nice, soft silicone buttons. You get that silicone membrane feel without the hassle of a buttons wells or membranes. The boards mount to the back case of the GB and mount using the existing case screw mounts. Note, that you won't be able to use a cartridge holder with these but you can still use a dummy cartridge. This should work with Kites board but I haven't tested 100% yet.
There are two versions, one for two buttons and one for four. The caps used are the standard 12mm ones that come in a variety of colors. Only a small modification to the caps are needed and a supplied 3d insert for the caps is whats needed plus the small mounting screws.
Here are some preliminary photos of what I have.