Headliner insulation
Anything that is "closed cell" would work (closed cell = Styrofoam cup or bubble wrap or poker table foam, open cell = sponge, magic eraser ). I suppose you could use fatmat or dynamat.
Try your local HomeDepot for reflectix insulation.
Tech specs:
# Reflects 97 Percent of Radiant Energy.
# Does not promote nesting of insects or rodents.
# Lowers heating and cooling costs year round.
# Easy to install.
# Prevents mold and mildew.
# Vapor and radon retarder.
# Not affected by moisture or humidity.
# Eco Options approved product
# Nontoxic/noncarcinogenic.
# Does Not Require Protective Gear To Install.
# Does Not Compress, Collapse Or Disintegrate
# Class A Class 1 fire rating
# Durable and lightweight
# Fiber and formaldehyde free
# Controls condensation
# Environmentally safe.
it doesnt say how thick the stuff is but at $15 for 16"X25' might be worth a try.