I have not tried this, so I can't swear it will work.

Many RC planes use foam core wings. There are suppliers on the internet and in many cities. They can supply foam cores in many stock airfoils. Mostly NACA 00XX and 40XX. For a setup fee they can make published airfoil you want. The foils can be straight, tapered or strange. A example of strange is NACA 0012 transitioning to a oval at the other end. You can make complex shapes by gluing tapered section togather.

The RC model websites have LOTS of articles on how to cut foam or who to buy airfoils from. I am not getting invloved with that. However the hotwire method is pretty primative and uses blocks and lines. A winter use for your rigging?

Once you have the foam core in the right size, turning it into a daggerboard or rudder is matter of vacuum bagging glass, kevlar, carbon, etc over it. You could lay it up by hand if you really wanted to.

In terms of cost. I spent a week cutting out 38 in long NACA 0008 cores for a daggerboard. I later found I could buy two 37 in long sections for $12 plus shipping. I can assure you, that is the bargin of a lifetime.

Also, in terms of Marstrom Rudders. Once you pay for the setup for a foam core, it becomes almost a stock item. I would interested in splitting the setup fee for a foam core. Or if someone has cores made let me know where