I like the Reflect-X idea!
On our new to us camper this spring, I pulled the shroud and covered a bunch of the very thin foam covering on the side of the plenum with aluminum duct tape. It helped a great deal with sealing it and reinforcing the old foam.
I believe I am benefiting from the reflective tape, but had I thought about the Reflect-X, I might have added it, too. I was unsure how hot the metal of the compressor would get.
I'll be you could get about 80% or better of your results just by covering the underside of the top surface of the cover, though getting it to stick could be trouble :E
I'm putting your mod on my to-do list, thanks! :C