This is my version of the no-hardware JTFPV Floss 3.0 Turtle Mode Fin, as sold by Pyrodrone. This is a brilliant use of the Floss 3.0's top plate, and it's turning into my go-to fin for this frame. Thanks to JTFPV for this great idea.
I was initially going to keep this to myself, but I saw others post similar designs and I thought mine had a little different play on it. I designed the fin to print as a flat piece rather than a pre-bent fin, so that when you thread the two ends through the top plate and bend the middle it puts the TPU under a bit of spring tension. I've had pretty good luck with this so far, since the material stress keeps the fin taut and springy.
I've included one with a 'camera protection loop' like the original, and one without. As others have said, getting the two thin ends through the middle slot can be a challenge, but with a pair of pliers it's not so bad. It stands about 50mm tall in the middle of the loop.
Not a super challenging print, just rotate it to one of the flat sides and let it go. I haven't played with the balance of infill and weight vs. strength, but it's something to think about when you're printing it.