Before going to a truck camper, I towed a bumper pull toy hauler. The truck bed was always covered with a construction grade canopy to hold extra or dirty items I did not want to carry in the toy hauler. Mine used full length barn doors in back and side doors with windows on both sides. This allowed easy access to most of the bed without having to climb in. I had a choice of going with fiberglass or steel, but opted for .040 aluminum which gave it strength but was light enough for me install and remove myself. I don't think they still offer .040 skin (now .035) since they were bought by Leer.
http://www.centurycaps.com/?d=23&dt=58