I used an old Nike dry fit compression shirt I stopped wearing. It was black, elastic, breathable, and pretty much the perfect material to cannibalize into a bib extension. I had a local dry cleaner put a 2 inch patch on the back, under where the straps met. I figured that would provide the highest degree of durability since the seems were the widest here. It worked out well and I'm happy with the fit now. Quite honestly, you could probably use material from cheap athletic shirts from Walmart or Target. As long as the fabric is synthetic and has some elasticity, you should be OK.