This was supposed to be a temporary solution, but it turned out to be good enough so I didn't bother getting anything better. Cheap and infinitely adjustable.
Some PVC tubing, some rope I had lying around and that's it. It does bounce around when pumping the wing, but I just keep it under the thumb of my back hand and it stays nicely put. The knots are some abomination I cooked up, but a constrictor knot should work fine here.