This “lump of coal” container has a twist-off-cap that reveals a secret inside compartment. Give something nice to the naughty people in your life!

See here for video: https://youtu.be/QiAlc_ylb0U

The “loose” cap and container files are easy to open. Print the “tight” cap and container files if you want the lid harder to find and open.

Print the “cap” and “container” files as oriented with the textured surfaces down. Raise the objects 3mm off the build plate and add supports underneath. Cura Tree supports work well. The bottom surface will be a bit messy. You could clean it up with a file, or just leave it messy because it is, after all, a lump of coal.