I was going to use rip stop nylon for my Lightbridge 12", but in the end I bought the Astrozap shroud. It's made from cotton/ polyester, it also has at each end a plastic strip sewn into the shroud. This helps push out the material so it does not enter the light path.
I also spray 3M water repellant on the shroud, this prevents it from getting dirty and also helps repel water on those dewy nights.
It has draw strings at each end.
I used it through the winter with great results, it also freezes well in those -7 nights we had
. Cheers, John.