These are my current UDT fins. The spines were shaved down with an extremely sharp razor plane, and then smoothed over with 80 grit W/D sandpaper glued to a small wood block. I did the final hand sanding under the faucet to facilitate the process.
I tried to replicate the flex of an old duckfoot fin. I left it a bit stiffer so I can approach the final flex slowly. As you can see, the top spines are a bit taller than the bottom. I thought the fins might have a bit more "direction" when kicking downward with more top spine. I've never liked fins with the top spines trimmed away too much. They kind of wander side to side as you kick.
My UDT's haven't been shortened as of this time. They feel pretty good in the water, but are a bear getting over rocks. Since I'm not in the water these days, it's all academic!
The Med/Lrg size (smallest available) is too long for my foot, so the heel is padded with neoprene. The top/inside of each foot pocket has two sheets of plastic from a milk container inserted to stiffen it up, and then a piece of neoprene to add comfort.
I'm currently looking at various brands of shoelaces to add that final touch of in-the-water security!