I'm about to do the same to my '85
Burro (very similar). I'm going to attempt to attach a strip of Reflectix insulation onto the
fiberglass and attach a burlap ironed-on to a thick, flannel interfacing to the surface of the Reflectix. I'll remove any residue using a frequently rotated rag of acetone and apply a 3M contact cement to both surfaces. When both surfaces are dry, I will carefully align the surfaces and press them together. When the two surfaces touch, they are bonded and will have to be torn apart to move. I have yet to work out the concave issue at both ends but bought material that can be shaped and stretched to conform to the compound surface. I'll likely start by attaching down the center seam and working my way to the sides while stretching the surface of the material into the concave pockets. I'll give a full report when this theory is put to the test.