I don't see why a zip-lock bag wouldn't be ok. I think I've just used an A4 clear sleeve before, (years ago, though), the type you use in a standard lever arch file, taped on all four sides, they just slice it open to take the docs out. Things may have changed now, mind...
These seem to be pretty hard to come by in a lock-down, I'm thinking of using a ziplock bag. Any ideas on that?