don't know if this was discussed but HDPE round rails!
http://www.usplastic.com/catalog/product.asp?catalog%5Fname=USPlastic&category%5Fname=72&product%5Fid=20241
how well would this work? 8 ft's short, and it's probably super hard to attach to something, but it would probably slide way different than metal wouldn't it? it's a lot slicker.