The box is a standard Aluvan light truck body. I boght it already on a (smaller) trailer. To design the interiour I drew everything on Sketchup and got the plywood cut at the hardware store. The rest of the job is drill and screws. The first Problem was the fact that the door isn't designed to be opened from the inside. So I had to cut a hole and install a small flap to reach out and operate the handle when inside. The advantage is: noone suspects me being inside since the flap looks a bit like a repair patch. I like equipment simple, safe and removeable (who needs heating in summer?)