I had felting problems with the coat on my finns. I also missed some external parasites and they thought life under the coat was pure heaven...
I have the matilda, and that fabric is not breathable, in spite of what they claim.
These coats are not adjustable, so you have to keep several sizes on hand to accommodate wool growth.. that was expensive.
I like the fly sheet idea! They would most likely tear easily, but if they were ez to make and replace and cheap, it might be the best solution.
This year I'm going to try an August shearing, and then put coats on. I won't be using those matilda's though. I think I'll take it apart and use it as pattern. Guess I'd better get that sewing machine out! Does Joann's sell that kind of fabric?
BTW, I've heard that you should have coats off for lambing through weaning-
nursing lambs can get caught in the coat and strangle.