I realize that this is an old post, but I would like to share my thoughts on this topic. I think that there are so many variables that it is impossible to have a one size fits all answer. If you are just doing a short tow across town at low speeds ( max 45 mph ), I think it could be done safely even if it has a fiberglass roof. However if you need to go a longer distance especially on the highway, I would try to avoid towing from the rear with an aluminum roof and I would never rear tow on the highway with a fiberglass roof.
The rear roll up doors on a box truck are not wind and water tight like a sea container. The doors are just to keep the load from falling into the road when the truck is driving and or to keep thieves from stealing the cargo when the truck is stopped. Also the trucks were not designed to go down the highway backwards at highway speeds. This is just like towing a road tractor with fairings from the rear. It will act like a parachute and the wind will get past the door and put a lot of stress on the roof which is the weakest link, causing it to fail and break
I first learned the light duty side of towing when I probably could not reach the pedals of my father's and uncles' wreckers. My father had a 1977 F 350 with a No Mar single line sling and pan dollys and my uncle had a 1971 F 350 with a PJ Ortiz, this one had the towbar and he would use an old tire and 4x4's. I ran some light duty trucks in Florida during the 90's. My formal training is with the school of hard knocks.
Iron Brokers International