A few years back I bought a nice shelf unit made for holding animal cages. There really aren't any shelves in it, just the angle galvanized steel frame. Hubby had brought home some cast off grates from a bakery which fit on the open framework okay, not great but holds pots. Last year I tried to glue some plastic greenhouse material to the back and top of the framework which didn't hold very long but it got me through last season. I used magnets and a couple shower curtains around the sides and front. Kept the frost off on a couple iffy nights.
So this time I am using zip ties to hold the plastic on the back and top. I'll use another shower curtain around the sides and across the front. It's not the greatest but clear shower curtains are easily replaced. I've used those greenhouse shelves before and found the cover doesn't hold up well and the frame didn't last. Part of it was my fault, I wasn't real good about getting that taken down and put away when it wasn't needed. But with this I can leave plants on it all summer and only have to worry about watering them.