A few weeks ago I got a good deal on some really nice 11.9oz carbon fiber cloth. Finally got around to doing something with it... basically experimenting with making some tough, lightweight fins.
First, I made a crude two-piece mold out of some strips of wood glued to a scrap of thick plastic sheet. The inner edges of the wood are beveled to create beveled edges on the fins. I sprayed the mold with WD-40 to prevent sticking of any excess resin.
I cut foam core board to fit into the mold, with a small strip of wood glued to it along the root edge. I also inset a short strip of wood at an angle into the middle of the foam core fin shape, to give the lamination something besides foam to hold the middle together. I cut out two pieces of carbon cloth using the foam core as a pattern, making sure to allow plenty of overlap at the edges (but not on the root edge).
Then I lay a sheet of wax paper into one half of the mold, brushed on some epoxy, and laid in one of the cut pieces of cloth. Dabbed some more epoxy on, and then pressed the foam core cutout into it. Coated that with epoxy, added the other piece of cloth, some more epoxy, and then topped it with another sheet of wax paper. Squeegied it smooth, then placed the second half of the mold on top, making sure it was aligned correctly. Topped this with a sheet of plywood and piled on some weight.
Half hour later, disassembled the mold and peeled off the wax paper. Then just trim the excess cloth and resin from the edge. Presto! A rigid, light weight fin! Seems pretty strong so far but we'll see how it holds up in actual use. (I'm mainly concerned about recovery impacts, not in-flight stress.)
I'll post some pics eventually.