This GBA cartridge case is suitable for a single game cartridge and is sturdy, does not have shifting front and back shells and has a good lock that holds it firmly closed.

The size of the case is also made so that you do not need to resize the cover art (from http://www.thecoverproject.net/index.php) to weird dimensions.
Just scale it down to 158 x 66mm (see attached screenshot) and the the cover art will still look great!
Just like the original, but smaller.

Use double sided tape strips on the edges of the front and back shells only. Do not place tape on the spine. You should not stick it on the spine as it will shift the cover art when you open the case.

Close the case and then place the cover art. First stick it on the front, fold it over the spine and then place it on the back making sure to align it to the edges.
Now when you open the case, the cover art has room to bulge out.