Some guys, who are better fabricators than me, have shaped custom parts
out of foam by using the hot "wire" method. In this case the wire was actually a thin, wide strip of metal, bent to the precise shape that they needed, & fabricated into a jig.
After which, they figured out the correct amount of current
that was needed so that they could slowly feed the foam through it without maiming the foam. And then the foam was sanded & coated with laminate.
I also would think that carbon fiber makes sense, & am wondering as to the choice of biaxial. What's the thinking behind using it? Aren't the primary load pathways on such a board oriented more along different axis then +/- 45 deg.?
For more knowledgable feedback on this topic, you might try boatdesign.net As well as looking towards some of the one-design, & box rule racing
catamarans. The ones where guys almost exclusively sail on DIY built boats.
might know a bit about this topic, & the afore mentioned info sources.
Also, I'm thinking that unless you truly can't get a hold of one, you'll save money
a router, & building a jig for it with which to shape things. As the tool will cost you far less than if you goof, & destroy that kind of foam.
Though there's nothing which says that you can't build a jig for a power planer to do the same job, or a jig for some other tool for that matter.
I say as much, as when Kurt Hughes
had just recently finished building his personal tri for himself, when I first met him, he'd farmed out having the rudder built (well, rudder #2), to the pro's at Speedwave. I thiink it was Speedwave.
When I asked about it, his comments were along the lines of $ being saved vs. were he to try & build another rudder himself, in order to get it right, & then goof. Then it'd wind
up costing more than the pro built one, & not be as strong, nor fair on top of things.
As the boat already had the first rudder which he'd built, still in place. And he was hauling the boat to install the Speedwave rudder. That, & that for foils that see so much load & such high speeds, so that things need to be very precisely machined.