I found some really cool boxes abandoned on a loading dock recently. Their original purpose was mailing boxes for 16mm films. Since they came from a library, they got mailed around a lot. These are reusable cardboard boxes, but they're made more toughly than most boxes. They have straps, metal corners, handles, and they just look tough. I couldn't resist picking up a few to use.
The use that I had in mind was a laptop carrying box. Presently, I carry my laptop around in my backpack. It's sort of big, almost too big for my backpack's laptop compartment. Also, the zipper is partially broken and the straps are due to give out in the next few months.
The box, of course, is here. Also present is a laptop (Dell Inspiron 8600, with extra paint and stickers added). The primary material added was closed-cell polyethylene foam. This is the tough foam that some electronics are packed in; it doesn't fill the box but rather provides a superstructure for the item being shipped. It usually comes as layers, approximately 1cm thick, laminated together and then cut into form.
Cut the shims from the foam and place them around the laptop so it fits snugly.
A shot of the final box, open.