Rug pads might work. Good padding, good friction, not prone to water damage, cheap (find some from someone who bought a rug and the mat was to big, or here: http://www.crateandbarrel.com/family.aspx?c=1268&f=8484&q=rug+pad&fromLocation=S earch&DIMID=400001&SearchPage=1
I haven't used it for your application. But I do have a sheet of it wrapped around and ziptied to the snowboard bracket of my roof rack. The rack doesn't close tight enough around boards and they rattle around (Insanely bad design). Seems to have held up well out there in the elements, but I've only used the so equiped board rack 4 or 5 times.