Two identically-shaped pieces screw together to form a box. It prints without supports and works well. If you want to print both pieces the same color, I recommend that you print them separately because they'll come out cleaner. I also recommend that you place the Z seam on the inside of the piece, especially if you are using a shiny filament. The exterior will look smoother.

Scaling notes: If you scale the box smaller, you will reduce the clearance I designed between the parts, and they may be too tight to fit properly. If you scale the box bigger, you will increase the clearance, and the fit will be looser.

I've made another version of this box. The exterior of the two versions is identical. The other version has different STL files for the top and bottom and has a taller interior space. https://www.thingiverse.com/thing:4633902

Please post your makes! :)