Lid for F-Puzzle

This is a snap-on lid for [Ferjerez' F-Puzzle](https://www.thingiverse.com/thing:1666559). This makes it suitable for transportation or to offer the puzzle as a gift.

The lid can be put underneath the puzzle while playing.

To avoid that one would only be able to close the lid by solving the puzzle, and therefore giving away the solution to the next one opening it, I provided an extra space in the lid for the fourth F. Place the letters as shown in the (first) photo before closing the lid.