Might as well ask what kind of hitch you need. There's really no right answer, just a lot of opinions.
I loosely adhered bubble foil to the upper interior most likely to get direct sun using 1/16" foam tape, then spray glued 1.5" batts of rockwool on top. I also insulated the ribs with the same foam tape and some foam sill gasket. Not saying that's the best way, it's just what I chose to do, and it seems to be working pretty well so far.
I didn't think using the bubble foil as your my only insulation was a good idea, but plenty of people here do. It's not really designed for the application, so there's no "right" way to do it. Just be aware that you get no reflective value if it's touching the surface you want to reflect heat from, just a small r-value.