proofing large batches

Hey folks.  I've been baking for the farmers market for a while now, usually making 5 or 6 types of bread, and anywhere from 8-16 of each type. We have two ovens and lots of counter space, and i purchased a 20qt hobart a few years back, and all of this has helped to make the larger production possible.  I've been covering the large amount of dough during first rise with some of those humongous zip lock bags (not gallon, but bigger, made for storing clothes i think) cut into pieces. It works okay, but i'm not sure if they are 'food safe' and i wouldn't want anything that would react with the acid in sourdough and leach out anything icky.  Any large batch bakers have any tricks? I've searched for food grade plastic sheeting, but mostly what i could find were links to large companies who would make custom rolls. Seemed to be for some serious commercial production, and not at all within my price range.  I've thought about the large black 1.1mm unscented trash bags, but i wasn't sure if that would be 'food safe' either.  Any ideas?

thanks!

Chris