Not all poly is equally UV resistant. If your roll has branding information on it I would highly recommend checking that before using it. Cleaning up the pieces of the wrong poly is a huge pain because it fragments more while you're trying to pick it up.

If what you have lying around isn't up to the task, ask around at your local greenhouses for offcuts from their last greenhouse skins. The box stores will try to sell you rolls from the paint department, and that's no good for this. You could also try any local dance studios about tulle they didn't use - plenty of folks use fabric mesh to do their brassica low tunnels.