[Christ]

Unfortunately, although I've done this before, I don't have any pics. They're on my Server, which is Deep Six at the moment, and isn't scheduled to be fixed for a good, long time.


Things you'll need:
Measurements for your bed, inside width and length.
Sheet of 3/8" ply wood or similar material.
Aluminum flat stock, 3mm, 1.5" width, enough to make both long sides, and over the tail gate.
Fasteners, either screws, rivets, grommets, bolt/nut, etc.
Paint, or something to waterproof the plywood/material you use.
Strips of rubber or plastic "bed caps", if you already have something similar.

Optional:
Pair of gas hatch shocks and mounts.
Hinges.

Simple Instructions:
Measure the inside width and length of your truck's bed area... typical measurements would be something along the lines of 48"W by 96"L (Large pickup, 8' bed area.)

Cut your sheet of plywood so that it fits, but doesn't rub, inside your bed area's largest perimeter measurement.

Attach the aluminum flat stock so that no less than 1/2" of it is attached to the plywood, but no less than 1/2" is laying over the edge, either. (Usually, you'd just use 3/4 of an inch, or exactly halfway, so you have a 3/4" over hang from the sheet of plywood.) Leave the flat stock approx 2" short of the cab end of the plywood sheet, so the aluminum stock acts as a hinge point to lift by.

Glue the rubber strips down to your bed sides and tailgate top, to protect the paint and prevent corrsion. These can be sourced from old inner tubes, whatever. They're optional.

Lay the sheet into the bed, so that the aluminum strips are supporting it on the bedsides and tail gate. The lid will stay in place, even on some of the worse roads. If you often fly over pot holes and go off-roading, you'll want a fastener of some sort, though. On strictly smooth roads, and highways, no need to fasten it, the weight alone (and air flow) will keep it in there.

Hinges, fasteners/locks, and lift shocks are all optional, and will obviously increase the price.