Todzilla said..Clark Rubber do an aero-foil section black pool noodle for just this purpose - about $25 from memory. Failing that they also do a range of pipe insulation tubes that are a lot softer than a pool noodle - they do a great job but will break down in the UV a little bit quicker than a noodle (mine lasted about two years, and cost about $15 for two) – they're easily stretched over aero bars.
Couldn't find the link to the aero noodles... but here's the pipe insulation:
www.clarkrubber.com.au/products/33887p-pipe-insulation The stuff you want from Clark Rubber is the UV-Stabilised foam stuff
made for covering playground equipment. Has a foam layer with a PVC outer. It slides OK over ROLA-size aero bars. Being UV-stabilised it lasts much, much longer than pool noodle or the pipe insulation stuff (and comes black and a range of 'designer' colours

). The PVC outer makes it nice for slicing boards on and off.
Another good product is the Yakima rack pad - fits a huge range of aero bars, the longer version is about 30" wide.