As I was sewing these, I thought to myself that the Rocky Mountain coats would probably fit if only the front was a bit wider. So I extended the front of one and tried it out on my Ag grey ewe lamb, shown on the right in the photo below.
It seems to have worked. I'll probably extend the other two C's like that too. Hansel (on the left in the photo) is wearing a D which fits him well for now.
I'll try to get better photos in the daylight. I found it easy to get the coats on the sheep while they were eating. They get so focused on their food, I can just go up and slip the coat over their heads and then fiddle with the legs while they're chowing down.