I have used a part of a feather cut into a tail shape, paint brush bristles, or a fin cut from a thin silicone place mat or under sink mat. These don't noticeably affect the action on my baits.
I also make the tail fin slot wider at the end of the bait and cross pin the fin through holes in the fin that are bigger than the pins. The fin will flap with the motion of the bait. Similar to the tail fin mounting on the Black Dog G2 Shellcracker. See the post below.
https://www.tackleunderground.com/community/topic/38362-jointed-lexan-tail-what-effect-does-it-have-if-any/#comment-313700
It is possible to make a flapping feather or bristle fin. I have done it by laying the feather or bristles out on a piece of scotch tape with just the mounting part of the fin on the tape. I then put epoxy on the part resting on the tape. Once the epoxy cures, I carefully peel off the tape and epoxy that side of the mounting part of the fin. Once that cures, I can drill holes in the epoxy part and cross pin in the bait. Sometimes the epoxy cracks and I have to fix it with a bit more epoxy. It's a pain to do, but can can give some really nice waving action even when the bait isn't moving.