This is the Bolty, my first design on Thingiverse! Although I have seen similar designs after making it I'm sure you'll love it.
Bolty is a Boltscrew with a secret compartment. This can be opened by screwing the top off clockwise to make sure it's more difficult for someone to open randomly! I made sure the design is strong, easy to print for any printer and has plenty of space to hide your cash, secret notes or anything else in there.
Enjoy!
:edit Made an additional nut to screw onto the Bolty to the files! Thanks AaronK9 for the idea.