[D911SC]

I used 3mm closed cell foam. It comes in various densities and I picked the firmest one of the 3 types I saw. I think the higher densities look nicer when the door parts (handles, speakers, release) are screwed in - doesn't depress quite so much.

My original panels had an open cell foam which is less water resistant. If your panels are waterproof then it shouldn't matter so much. I sprayed my MDF with clear paint to achieve some sort of waterproofing.