Cover for the battery compartiment of old Nintendo Game & Watch games.
I found another one on thingiverse made by "wizard23" in 2008:
Did not test this one out, I created mine before even noticing there was one already :)
Designed and printed @ 0,075mm, did the job just fine as you can see in the pics.
Hopefully I can help someone out who lost some covers.
Made using Sketchup.