From experience I would think that the heat buildup is not the real problem. Working with the "66 Road Car", we quite often had to have the rear hatch open to prime the webers as he only cranked it every once in a while. What I observed and probably is what you should be concerned with is soot buildup on the underside of the hatch. It isn't that hard to clean, but if you don't have something easy to clean between the fiberglass and the exhaust it just may be. There are some places that water can get that may play havoc with the electrics on the rear hatch. So I would opt for some aluminum or stainless to deflect the exhaust. Better yet would be to do as the chassis dyno shops do, and that would be to put some flex tubing to the ends of the exhast to let it keep the exhaust away from the fiberglass. If you are in the garage, it keeps the buildup of gasses away from the work area. Carbon monoxide will give you a killer headache when its content becomes significant. I remember cutting a concrete slab in the basement of a house cut into the side of a hill. We ran fans to the room but would have to take breaks every 10 minutes of so due to the nausea and headaches.
Bill
The angle of exhaust exit?:
