Those ARE DS cases
Seems I forgot to
upload the picture of them.. Basically you use an x-acto,razor,whatever small knife based object you have and cut the bottom part of the GBA slot and the top half of the DS slot
like this. With that, the remaining parts of the GBA slot will perfectly hold GB/GBC games upside-down. If you want, you can cut the top part of the left half of the case and the instructions will fit in there as well side-ways.