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12 Sheets 12" x 12" x 1" Black Polyethylene Plank Foam 1 5PCF Best Price PE | eBay this looks like the same stuff

You would Want a hard foam, probably urethane but I found this used for packing in some equipment that was shipped to work, don't have any left sorry!

I use old cigar boxes and line them with closed cell foam that I scavenged from some equipment packing at work.241259241260
It works out good for me. ..

Beautiful job, but you may want to consider using closed cell foam if you are going to use foam, that way it won't absorb any moisture.