I always wondered why the default (usually unadjustable) thumbrest is always convex shaped on the thumb side - contentrating the pressure of the weight on a single spot, usually where it hurts most within the shortest period of time.
I emptied countless bottles of wine to find a perfect piece of cork that could be shaped into a biconcave pad, spreading the weight onto a relatively large area. It's not a replacement for a Kooiman thumb rest, but it costs nothing and can be made within some fifteen minutes. Be sure not to make the cork too thick, else you'd have to move the whole thumbrest upward.
the shaped cork, glued directly onto the existing thumbrest padding.
the new thumbrest in action. Much more comfortable!