For tail fins, I put a dab of epoxy on the inside portion of a plastic milk carton. I then add some fin material (I use DNA Fish Fiber from Cabelas). I then sandwich the fibers between another piece of milk carton, leaving the fin material sticking out. Make sure the epoxy penetrates through the thickness of the fin material. I can then carefully shape the fin material to my liking, and leave it to dry. I use 5 minute epoxy and allow it to sit for 30-60 minutes.
The epoxy will not stick to the carton material. After it is dry simply peal it off, and you have a fin. Both ends of the fin can be trimmed as needed to properly fit your bait. Hope this makes sense.