3D output might make a good plug to then craft the tool/mold for composite layup of parts such as blades, fins and doo-hickeys.

I've thought about it a couple of times--yet another project. I haven't even unboxed my laser 3d printer after shipping it back from Maui.

There's no strength issue for a SUP fin, especially for flat water. If there were it would be simple enough to design the fin as a core and bag a layer of carbon on it. Biggest problem is bed size. For my printers I'd have to do the fin vertically or at 45 degrees and that's a time sink. But then who cares. The printers are idle anyway.