I used heater duct insulation; foil faced self adhesive, 1/8" thick. Big box store for $18 roll...very happy with the results...not as good as Dynamat, however, much cheaper. I put it on all my aluminum panels, behind dash, under mats, under hood, everywhere that isn't exposed...much quieter/warmer D-90, panels sound solid, and non has fallen off in a year.
This is the stuff I used everywhere on my truck, fom engine bay and hood to headliner, footwells, doors, bulkhead and seat boxes and everywhere else. This is a good price for 100Sq/ft. It made a huge difference in noise and heat reduction.