Annular fins in theory should generate more lift in light wind conditions for less draft than a conventional fin (think of it as a low draft formula fin).    Higher aspect ratio fin like this should in theory have less drag.   Uploading this design as worked better than the beefed up 25cm version posted previously.   When I mean worked better, it performed better as a fin generating more lift (seems a symmetrical foil is superior to a cambered foil) however if failed (bent) structurally at the strut to ring interface.

This design will work if add a layer of glass and/or add a stiffener plate to the ring as per the 25cm design.  Note it has a deep tuttle connection to suit my big board.

Printed in PETG, contact cement it to the steel ring (in my case an old gas bottle), coat in some Polyurethane then sand smooth.   

I used Procast Rigid Polyurethane https://www.flowresins.com.au/products/procast-rigid-polyurethane.

Please note it may be difficult to reproduce yourself as relies on having a similar diameter steel ring.   You may get lucky.  Hopefully the design shows how to make your own variation.   It collects weed/reeds/debris like crazy so forget this design if your spot is not clean water.

Video of it in action, forgot to turn off the GoPro stabilisation stuff so fin gets clipped in shot but fin seemed to work just fin given the very limited wind.