Inspired by several of 'ball in box' designs already on Thingiverse, and especially by someone mentioning the idea of putting something in the box while printing (I haven't been able to find that reference again) I've designed my own version. It's fully parameterised on OpenSCAD.
The sizes I've chosen suit an Australian 20c coin.
The side portholes are elliptical to make a little easier to print.

Made on an Ultimaker using 0.1mm layers and a 'brim'. Pretty straightforward to make, I think.