Fluke Shape Retention For Homemade Tail?

[Discordien]

I'm working on my second tail, and I did a test run for it. It swims fine, but I realized that the fluke shape doesn't quite hold itself in the water (if that makes any sense). It might not help that the shape doesn't exactly resemble the monofin (Suntails), but I think this is a simple fix.
I think I may need to make some kind of casing/insert to keep it at the shape that I'd like, possibly one made of marine vinyl. Any advice? Here's a picture of my tail.

[Mermaid Jaffa]

I'm working on my second tail, and I did a test run for it. It swims fine, but I realized that the fluke shape doesn't quite hold itself in the water (if that makes any sense). It might not help that the shape doesn't exactly resemble the monofin (Suntails), but I think this is a simple fix.
I think I may need to make some kind of casing/insert to keep it at the shape that I'd like, possibly one made of marine vinyl. Any advice? Here's a picture of my tail.

Do you mean that the fabric on the fluke area rides up when swimming, and exposing the monofin?

Sew some plastic snaps on the fluke edges to hold the two sides together.

[Discordien]

No, the monofin riding up has never been a problem. I think the best way to describe the problem is that when I put the tail in water, the fabric..er, collapses? I made a "sock" for it, so the printing for the monofin doesn't show up as much, but I was thinking a casing of some sort would help to "smooth" the shape out. You know how some companies like FinFolk have their casings? That was what I was thinking of to fix this problem. At least, I think that's what the vinyl casing is for. Am I mistaken?
I didn't take that with how thin the fabric was or the fact that my monofin would show so easily because it's black (opposed to the yellow fabric), so lesson learned there.

[Mermaid Jaffa]

Yea. I make light colored tails too. When I bought my Sun Tails, I made sure the monofin is the light blue one. Blue disappears in swimming pool water. Green would disappear in a lake or ocean. As for the fabric collapsing, I think it gives the tail some character. You know like how a goldfish has soft flowing fins. I hope that makes some sense!