The first thing I need to find out is how to search for sheep coats in a way that google recognizes. All the variations I get are about coats from sheepskin and sheep costumes.
Maybe you've found something by now.
Here's a couple sites that might have ideas.
Seems like a cotton t-shirt might be the right weight and stretch, modified somehow?
Natural cotton-duck sheep blanket with neck and mesh butt insert, designed exclusively for show sheep. Sheep blanket fabric allows wool to breathe and keeps lambs clean and dry until show time. Single belly strap has D-rings for adjustment, to keep sheep blanket in place. Sheep blankets measure 36" wide.
This is made with the back of a man's workshirt as the main part. It looks good on the sheep. She seemed comfortable in it, but the rest of the flock completely freaked out and tried to expel her.
But at least now I understand how it all fits together.
The plan, make 2 adult size and two juvenal jackets. Apply them at shearing time and then discover if the fleece really is better with a coat. If yes, then coats for everyone!