Ed, I don't understand the garden hose thing or the cardboard. If the fiberglass is rather thin, just rough up (60-80 grit minimum) and lay more mat. If i'm building up thickness I use the heavy (Evercoat Sea Mat?) and laydown two layers at a time. If it needs more, make sure you rough up after it has dried or it won't adhere.
2" should be pleanty of clearance on exhaust to glass, unless you are next to a cat converter. I had a muffler shop build a complete exhaust on my 69 coupe and I kept smelling a funky burn smell on long trips. Years later I noticed he was 3/4" away right under my trottle pedal, no wonder my foot was always hot! When I moved it down I noticed it had kinda melted that area just a bit, must have gotten luck on the fire thing