I didn't know these existed? I suppose they are only for select fins?
www.select-hydrofoils.com/en/shop/windsurfing-accessories/multiconic-adaptor/

they are, but it is possible to machine the base of a fin down to then make one of these fit

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Mark _australia said..
they are, but it is possible to machine the base of a fin down to then make one of these fit

Agreed. I used one to convert a fin. I already had one of these in a trimbox fin, so I just copied the profile using a grinder.

Sue, I have one of these, I will show you next time you are down at the lake

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tbwonder said..
Sue, I have one of these, I will show you next time you are down at the lake

I wonder if I can convert my 27cm powerbox fin to a US box..I'm keen to see it..

27 cm length, would be at or near the strength of a US box, using an adapter IMO make it even less sound

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sboardcrazy said..

tbwonder said..
Sue, I have one of these, I will show you next time you are down at the lake

I wonder if I can convert my 27cm powerbox fin to a US box..I'm keen to see it..

That's would be the wrong direction for a basic conversion box, you can easily go the other direction though, US to Powerbox.
You'd have to do some serious grinding and reshaping of the powerbox base, to go into a US box, and then you wouldn't even need the conversion box.
I've been thinking about getting a trim base conversion box, and adapting a US base fin into it, which would require cutting, drilling, and then adding epoxy to fill the voids w/in the trim conversion box, after installing the altered US base fin. Doable, but not easy, those are designed for Multi-conic base fins only, not US base fins.
I've got an older trim box board that's perfect for very light wind conditions in the ocean, but finding a trim base fin designed for ocean use is basically impossible now days. Plenty of trim base racing fins still available though, but I don't want or need one of those kelp grabbers, as well as weed fins, but don't want one of those either.