Bolster backing material - " whaler board "

[McGillicuddy]

Not sure what you found but if it was seemingly heavy, it was more likely a HDPE based product. Trex, Everwood and a number of other plastic decking suppliers have similar products but are HDPE (high density polyethylene) - think cutting boards and starboard - a totally different plastic chemistry.

Like the Azek product, Komatex is another lightweight pvc foam that would work well for bolster or combing backing.