Does anyone know if you can get US box fins as large as 38-40cm?

It would need to be a well fixed box, to stop it rolling

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decrepit said..
It would need to be a well fixed box, to stop it rolling

Yeah that was my initial thoughts. Might be a bad idea because I want to try one in my naish inflatable windsup and the us box slot is just glued on to the pvc, Might be too much strain for it

Yes , you can get fins that long. I have a 42 in my old mistral race board. Its only rarely used in light wind. I wouldn't want to push off it.

Wouldn't trust one that big.

Why do you need?

Because big is better.
What are we talking about?

Wonder if a proper glass surfboard fin would be better, like this one. The fin it comes with is very flexible plastic.

I use surfboard fins in all of my boards, sups too. Some surfboard fins are too thick, some are good. I use fins from need essentials which are excellent.

avoid the thin tipped longboard fins. Too much tip flex. Look for the fins with more area in the tip like the image you posted or the whale fin need essentials sell.