If you have creaking in your shell, try this: grab some thin film (I used 1 mil kapton), and shim the back util you get a nice tight fit. The black foam pad is suffering from compression set, I suspect, so bulking it up adds some compressive force. Be careful to avoid the contacts for NFC and wireless charging. Also, I would avoid metallic materials as this might interfere with charging or communications.

Hope this helps!