I've got a bunch of boxes from 3d printers and other big stuff that are sturdy and since I'll be moving soon, I figured I might save them. Since they do not have a handle, this allows you to add it. 2 pieces, snap together. They are made for ~6mm cardboard thickness, but will work with thiner boxes, too.

There's also a stencil I used to draw the cut line on the cardboard box.