Every household has these Variera boxes (also called Updattera for reasons best known to the Swedes!) They are fantastic for holding loads of stuff but.

They don't stack!

Well now they do.

These slide over the edge (3 or 4 needed per box) and the groove holds onto the moulded ridge in the base of the box so they don't slide around

Creality

Ender 3 Pro

Doesn't Matter

No

15% best for strength

at least 10% infill needed (you're supporting the weight of the box above)