Bill,
Mine are metal spacers that are about 1.25″ by 1.25″. flat against the bracket but convex against the bumper.
The pieces have a pretty good crown on them, so if I had to make ’em I’d get some bar aluminum .25″ thick (too allow plenty of thickness for filing the front, (back?). Do the whole bar at one time then cut off individual spacers and drill the holes.
Takes some time but that way you’d know the coutour would match the bumper and prevent any damage when you tighten it down.
My 2 cents for the morning,
Ringo