A good closed cell foam should work just fine. You could probably also use a roll of fiberglass insulation as well. Either way, you just want something that will keep the cold away from contacting the lid, and won't absorb moisture.

I like your whole design idea. Very thorough, IMO. I f you relly want to save $ get the foil backed foam insulation, then no urgent need to cover with plastic ; the foil is quite efficient.

which might not have been a problem at the factory, where the doors were assembled away from the main units. The truth is that fiberglass isn't as good an insulator as styrofoam or even plain old upholstery foam. I'd use either of those.

IOW, don't use what they used if you've got something better.

(I thought this would be an "I wrapped the top in bubble wrap and it's doing just fine" post)

never thought of that stuff.