I use a microfoam product that has a thin plastic backing when I need to protect my bulb beds temporarily during severe cold snaps, but I wouldn't leave it on all winter because of the danger of rot. Some years ago a friend who had a large nursery operation gave me some samples of a product that is made of plastic but has very tiny pores that allow the passage of gases but not water. He used it to cover propagating beds with cuttings in them. I used this successfully for covering cuttings both under glass and outdoors. I don't know the name of it, but maybe you could search under "micropore" and find something similar. This product is thin and would need to be supported so as not to lie right on the cacti. Regarding glochids, you can avoid them by wearing latex gloves. If they get in your skin, you may be able to remove them by using a hot waxing product sold for hair removal (find it in the cosmetics department). I didn't have any handy when a little cholla (Opuntia sp.) jumped onto my pants leg last month in Chile, and a spine or glochid left me with quite a sore spot. Jane McGary Portland, Oregon, USA