When working with balsa skins, the wrinkles are never a problem for me, I buy the cheapest drop cloths, maybe 1 1/2 mil stuff, I would image thicker plastic sheet would cause impressions where wrinkled. My opinion is buy the cheapest bag and throw it away after one use. The bag is only keeping the air out, the atmosphere is doing the work, better and thicker bags just cost more.
Tony Hallo