I lack a truck-- there was only a Sable in my stable when I built this, old enough to
smoke, and it did. I've since replaced it with a Geo, still only a sedan.
So, how could I go to my local hardware store and bring home some 4 x 8 sheets of plywood, for example? Below you will see my solution.
It's a simple wooden frame. The front end lies on the roof, secured with some baling wire to the bolt inside the back door that holds the top of the seat belt (finally, I've found a use for seatbelts). The wire doesn't interfere with the back door.
The vertical supports at the rear rest on the trunk lid. My car has a 'luggage carrier' on top of the trunk, so the support is baling-wired to that. If you don't have one, a couple of eye-bolts would serve the same purpose.
Use old pieces of carpet or foam rubber to protect the paint where it contacts the surface. You can cover the wire with plastic tubing so it won't scratch anything.
If you have to buy the stuff to make it, it would probably be under $20.
Baling wire is actually (in urban areas) re-bar tie wire, fairly cheap at the hardware store. Fortunately, I had all the materials laying around my back yard.