various materials used for covering uppers to protect them from damage during bottoming or finishing
Sacrificial shoe covers are often inseamed into the shoe, then trimmed from the outside. This leaves a small layer within the shoe, invisible from the outside.
Plastic
Commercial covers are now often made of plastic. Plastic covers may also be made to shrink under heat, or for fitting in a vacuum chamber.
Fabric
In the past, makers would sometimes use scrap fabric, such as old bedsheets.