Kevlar and carbon, but no decent foam to find?
No, polyurethane is good to start with as it is the only foam polyester works with.
Polyester is cheap and therefore better to start building your first board.
(it bites holes in other foams, so only use polyester with polyurethane)
Unfortunately polyurethane is not as strong as other foams with same density...
Strenght is for the biggest part a result of the layers you put on your foam.
Much glassfibers + polyester = strong, but heavy
1 layer of carbon +epoxy = very strong, less weight, 3 or 4 times more expensive(?)
Makes no sense to combine Carbon and polyester if you want to reduce weight.
Try it cheap first. Weight will be a bit more, strength a bit less, but you'll be able to build a quite decent testboard for half the price of an epoxy-carbon board.
If you build it with plywood, strengt will not be a huge problem if you make multiple layers...
And if you want, you can always reinforce with a layer of fiberglass and polyester.
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Hope this helps.