I've used an invisible mending cloth on badly damaged Graflex bellows. This cloth is designed to repair suits etc and goes inside despite its name it comes in a variety of colours, the black version was light tight, and is very thin with a strong self adhesive on the one side. I used it inside and outside and it had no impact on the way the bellows fold and compress and it's very difficult to see where the repairs have been made.

The type I used is no longer made, the company in Holland ceased trading but there are others on the market.