This approach is pretty simple. Just lightly hot glue strips under the yak, regular glue in between, and in line w/ the strips making the recess. Cut 1" off the end of ea strip and make a short upstand to fair into the coaming upstand. After glue sets, remove ea brace, sand up, glass and reinstall - glue , hot glue or contact cement

Option E is similar except a hook is mounted on the i/s of the yak and velco is mounted under the coaming base. The velcro is mostly in compression so it may work as a moveable thigh brace.

Another idea for fitting is to use Pete Ford's compass mount approach - spray glue sandpaper to the u/s of the coaming and rub the brace block back and forth to get a better fit. If the curvature of the coaming is similar for a few inches ea side it may give a pretty tight fit.

-mick