Step 1: How it works
Step 2: Materials
1 sturdy box. My city-issued recycle bin was perfect for the job.
Plywood or chipboard (OSB) for the base.
Some scraps of wood for the box hinge (2x4), side rails (1x2), and prop (1x2)
Screws to assemble (for example #8x1" and #8x2" wood screws)
2 hinges, and screws for mounting (for example #8x1/2" wood screws)
1 screw hook
1 clamp-type paper clip
1 small keyring
And, a saw and a screwdriver.
Of course you can improvise with whatever materials you have on hand.