Cash:
I retrofitted strip knee braces like Kurt's into my Great Auk for my wife, as she was having trouble locking in well enough to roll it. Mine are curved (I molded them around a piece of half-round landscape timber)to hook the knee. Make sure that you take into account the angle at which your leg will contact the brace and have the brace mirror that angle, otherwise you can end up with your leg contacting the brace on an edge. Not good. I lined the underside with sticky neoprene (old mouse pads work well). You don't want to use nylon covered neoprene, as it will be very slippery, especially if you wear a wetsuit while paddling.

Regards,

Marcel