On mine, and a couple other late '80s that I've seen, the fiberglass mufflers were strapped to the side of the bilge with what appeared to be pieces of the plastic rub rail insert, or something similar. I would think that a rubber bungie cord would be an acceptable substitute to use. If they are getting hot enough to cause issues with either of those materials, then you have another issue going on that is causing too much heat. The mufflers should be fairly cool to the touch as there is water exhausting with the engine exhaust.
1988 Tristar 190 Sport, 351W w/GT-40's, 1:1 Velvetdrive