I am new to this forum but know a bit about insulation (18 years at Owens Corning). Would a 3/4" thick for pmable glueable sheet work? We make this stuff for hvac that's semi rigid, cut able, has fiberglass facing with foiled backing-- that combo provides resistance to reverb as well as Eliminates sound transfer...also has R 3-4 so will reduce thermal heat transfer...
That foam stuff in my 96 is falling off and until I read your posts I hadn't thought much about it..
Happy to send you a small sample