Here's an even tougher version of my previous design:
I've strengthened the clips and added guards to protect the clips if you drop it. I also added a lanyard loop.
This prints without supports, each half flat on the bed, as shown in the image. The clips extend from the base at an angle of 65 degrees, which should be fine for most printers.Make sure you have good bed adhesion.
This works great in both PLA and PETG. (This is the first thing I printed with PETG, a tip for anyone who hasn't used it before: If your cooling fan is set too high, you won't get good layer adhesion, it took me a few tries to figure that out.)
I can now change the proportions quite easily. If you think I should make one in a particular shape/size to suit a certain item, let me know.
Doesn't matter Rigid
When printing you will have the obvious trade-off between weight and how rugged this box is. I had good results printing this with a 0.4mm nozzle, with 0.1mm and 0.2mm layer heights. Much larger layer heights might reduce the performance of the clips, I don't know.
When printed with 3 perimeters and 30% infill, this feels rock solid. You could probably get away with 2 perimeters and less infill for many purposes.
I suggest using 4 or more top and bottom layers, to ensure the surfaces are well sealed.
The box in the photo is printed with Amazon Basics PETG.