Shimmer that is a cracking idea. Thank you.I've used sheets of expanded polystyrene from Wickes. Easy to cut and fit between the battens, then cover with some thin external quality (so it doesn't go black in the damp) ply. Get some bigger sheets delivered that mean the least cutting to fit the shed. You can also paint inside when it's done. Any leftover household paint would do to protect the ply and make it easy to wipe down.