To get more stiffness than foam can provide try to find an old plastic drum or a beyond repair old kayak and cut out a couple of pieces with the curvature and size that you want. Attach these to the existing thigh braces to make them more "aggressive" and then add thin foam for comfort. You might be able to attach them with just contact cement but drilling a couple of holes and bolting them to the existing thigh braces would like be more durable. Good luck.