This is a shell cover for the WonderSwan adapter PCB created for the Cartreader.
Two P-Tite screws with a nominal diameter of 2 and a length of 8 to 10 mm are required for assembly.
 (M2 tapping screws will probably work too)
You can also protect the WonderSwan cartridge slot by making a cap.

There are two WonderSwan adapter boards, v1 and v2, and please note that they are different sizes.
v1/v2 are distinguished by the file name.
The same cap can be used for both v1 and v2.