the files here are made for 2mm thick walls for the box, but you can change the global scale of each part to change that

it is mostly made for cardboard walls but other soft materials could work
the assembly cohesion is made by ribs that goes in the walls so it need to be softer than the plastic corners

the top caps can be a little bit hard to remove, so it is better to sand it before pushing it in the first time
(sand the end sphere to the point where you can't see the printing stripes on it)

you will also have to cut all walls to the dimenssions you want, and cut extra holes in the bottom "wall"
those holes are circles of Ø40mm (for 2mm thick walls)
if you have access to a lasercutter, it could help you a lot
otherwise, use scissors, and i would advice you to first cut all the outter walls and assemble them together, then the inner walls by aiming a little too long to have the least gap possible, and then bottom and top

printing them with an ultimaker² take ~1h per corner