Neat solution Detlev but difficult to see how much the nylon is proud of cross member. Mine is hard 66 grade nylon block shaped to tightly fill the upturned U cross section and sitting only 2-3mm proud of member edges with tapered edges front and rear. Main reason for choosing the medium was it's slipperyness so avoiding transmitting too much force to chassis when in contact with road obstruction.
BTW holes make sheet metal objects stiffer and mine are only 6mm anyway. The mounting bolts are simply to hold block in place, it's tight fit taking all the force.
