You will need to turn this .stl into a “hole” in your 3D design software to work.
Designing:
Current size fits any decking screw or #8 button head screw.
Make sure the smaller slotted side of the keyhole is flush with the backside face of your design. The larger portion of the pocket will be imbedded within your design. You'll notice for my demonstration of a “7” I had to flip the design backwards to flush mount the keyhole to the back.
Many recommend having your keyholes mounted vertically with the smallest portion of the slotted hole at the top. My demo piece just didn't allow for verticals easily.
Printing:
I print my files with the keyhole facedown on the print bed, no supports should be needed. My printer was able to easily cross the gap without issue at the current size.
The author hasn't provided the model origin yet.