Keystone Slot (tight)

The original Keystone Slot from @domrim_121160 had a few issues for me.

• The Keystones didnt really click in.
• They came loose very easily when pushing on the front.
• The overall size with 5mm borders was too large for my taste. It still works standalone with 2mm and is supposed to be included in different designs anyway.
• Some dependencies in the drawings didn't make sense (changing them altered the geometry in an undesired way)

So I fixed that and now my keystones click in very satisfyingly and only release if push on the latch.

Also: thank you domrim for providing the .f3d file. It would have sucked otherwise.