I have a flat top aluminium ' Mountain Top ' for my Ranger which sounds like yours. I'd only really insulate the underside of the top as that's where most of your heat is going to go. It will make the inside of bed wet though with condensation. I'd avoid covering the floor or the arches though as the really need to breathe - assuming you don't already have a load liner fitted ? Spray on Bedliner from somebody like Buzzweld is pretty effective at sound and thermal insulation and it's pretty tough too.