I had the same question, thought about making my own, but finally bit the bullet and ordered a pair from Sunset Imports. As I recall, it was a lot of money for a couple of straps but they work good.
I bought a pack of velcro straps that had the plastic buckle on one end and trimmed to length (they were about 2' long) from Home Depot, think I spent $10.00 and they worked great.