I buy that double bubble stuff in large quantities, up to 4000 sq. ft. at a time, to put under concrete floors that have in floor heat. I never thought to do the panel with it, but it would work well. I have 'Agri-plus' fiberglass insulation in my work van, it's 1.5" thick and has a white vinyl facing on one side. Sorry, I can't supply a link, but it has an R value of 5.
