I removed the front plate and bellows last night. The leather was incredibly thin and it did not seem to have an inner layer. Mine was torn in many places. I will have to reassemble the pieces to make a pattern.

Another possible source for an inner layer is to use a piece of the packaging from sheet film. It is the light proof black plastic envelope that is inside the double box. It is shiny but it won't have an effect due to the inner horn restricting the incoming light to a narrow strip of the film.

Whatever is used will have to be very thin and flexible so as not to restrict the movement of the lens.