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I don't know much about sourcing, however I do know a little about the core material. If you are looking for strength you want to go with a material that is strong in compression. Which means it doesn't squish easily. I would suggest going with something other than EPS foam but I have never tested it as a core material. Honeycomb nomex, for example is what aerospace companies use in their products but you shouldn't have to go that far. Some denser foam like EPO should work