The rubber I had was ok, but a little too floppy to create a firm grip. I had some 0.9mm thick TPU sample film called Tuftane which was perfect. Can also just use duck tape to secure the washer to the inflation bag. I have a cheap compass cutter from eBay which cuts perfect circles – definitely recommend investing in one of those.
https://www.ebay.co.uk/p/Jakar-Compass-Circle-Cutter-6-Spares-2-Leads/1606249292?iid=292051332007
