My local marine store has good quality black neoprene (sticky backed on one side) foam 10mm width in various thicknesses, I bought some 8mm, that would probably do fine for what you want, I suggest you make it down to your local marine supplies store.
Depending on how "pretty" you want it and your resources, you could also build something from brass, PVC, or machined aluminium. See http://www.pbase.com/benhua/dr125_ed80 for an example.
Thanks for the advice Mike. I'll make sure make the foam is thick enough to apply enough pressure on the tube, but not too much to dent it! I'll do the "point at zenith" test, as well.