Urethane cord is used to make heat fused belts in several profiles
including round cross section. There are several companies selling
material as well as heat welding systems to join the ends, although a
vee shaped jig and a hot knife will do if one is careful. A search with
Google http://www.google.com/  on the web should yield several sources.

Spencer Chase
http://www.SpencersErolls.com/