I don’t see a need for a backing on the SS’s. As mentioned, they swell when they take on water. After 5 minutes of so they stop swelling or stabilize. Try this. Soak one for five minutes and lap it flat. Let it dry overnight and soak and lap it the next day. It will still be flat becaus you soaked it. Let it dry over night again. On the third day give it a quik rinse and lap it immediately. Now it will have a low spot in the center because it hasn’t swelled with water yet.

I don’t submerge mine to soak them either. When I’m on my 1k I pool some water on my 5k and so on.