In the repair video you mention using a 3mm thick rubber, which is similar to inter tubing. Would something like this work?
Stock 3mm x 12" x 12" Silicone Rubber Sheet Self Adhesive High Temp Plate Mat
This probably wont work because is is silicone rubber. There must be a million sellers on ebay with 3mm thick rubber sheet. you can always go to a tyre repair shop and ask for an old inner tube. I am sure they will be happy for you to take it off their hands.
I didn’t think about old tire tubes. Not sure if they are common enough around here, but I will check for sure, thank you.