Hull chock material

Does anyone know what type of foam hull chocks are typically made of? I would like to order some for other projects but can't identify it.
Polypropylene closed cell foam, not sure of the density though....it is offered in various densities

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Randy Neubauer
Apple Valley, MN
2001 H18M
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A bit expensive, but these folks have just about any type of foam you could need:

http://www.mcmaster.com/#standard-foam-sheets/=103cjsu

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Hobie 16 (3 formerly)
MacGregor 25 (formerly)
Chrysler Dagger 14 (formerly)
NACRA 5.0 (currently)
High Point, NC
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i use trow able cushions on my f16