This is a box designed to hold Mainspring Clamps, such as the 4-Piece Set of Flat Mainspring Clamps sold at https://timesavers.com/i-24048584-4-piece-set-of-flat-mainspring-clamps.html
When I started learning clock repair I found the first tools I needed were a Let Down key set and a set of Mainspring Clamps. You use these tools to safely release the power of the clock's mainsprings, so you can safely proceed to disassemble the clock.
This is a simple box with one side that slides open. That side is normally kept closed by gravity (see the photo). I added a simple image of a clamp on the top so I can tell the box holds mainspring clamps.
Print with the open part of the base facing up (as in the STL file). Print the sliding side with the thumb hole facing up (as in the STL file).
I like to print this in PLA.
To assemble the box, slide the side of the box into the slot in the box. Make sure the round part of the side's half-moon thumb hole points toward the bottom of the box. See the photo for correct assembly.
The side may be a tight fit at first. Slide it in and out a few times to loosen it. It's designed with a clearance of 0.5 mm.
The sliding side has a modified Dovetail joint to hold it securely onto the box. I chose to make a sliding side instead of a sliding lid so gravity will keep the box closed.
See the Fusion 360 design file, MainspringClampsCase.f3d, to see the details of construction, or to adjust the size of the box.