I agree with this, but I would prefer metal for clamps. The upward pressure on whatever is holding the clamp and the need to move it around easily make metal the better choice for clamping.
However, I am in the process of making another Fritz und Franz jig, this time with aluminum runners for the slot. My wagon's slot was close enough to 5/8" that I ordered square aluminum bar in that size and sanded it down to a perfect fit. A couple of tapped holes will attach the runners to the jig. Not hard at all and no expansion or contraction to mess with the fit.