Thought you'd appreciate this pic. I've re-worked a couple of pairs of new formula UDTs this year and discovered that variable speed angle grinders do a remarkably good job of tapering fins. Use the right blade and be careful not to overheat the area being worked as the rubber has a tendency to liquidise! The UDTs in the middle replicate the dimensions of of my old ducks (right) but are way more powerful. The GG taper and flex modifications provided most of the inspiration and clues for me to follow. Happy matsurfing!