I've been able to get some experience with these fins that I made. I've found them to be very brittle. I've broke a couple of them now. One I broke when tapping the hole. Another broke while mounted on the board. I didn't knock the fin excessively. Probably need to add some glass cloth to the formula.
yeah, resin alone is very strong, but extremely brittle. i used a big chunk of Delrin chopping block (got mine from walmart, comes in tons of colours if you look around for it) that's about half an inch wide then cut the fins to rough shape, followed by fine shaping on the belt sander. the fins have some flex, but not enough to distort the riding in any way, perhaps the flex even does some good for the board. i'm currently using fins that are approximately 1 inch by 5 inches on my 128x40 board and they are working just fine. the fins are prone to gouges if you ride over hard objects, but they will just dent or have strips peel off which you can fix with a utility knife.