My recommendation is flexible aircraft dope, as is used to cover cloth wing fabric on older (and some modern) airplanes and sailplanes. If it can hold up to 100+ mph airspeeds, it can probably hold up to photographic uses...

You can buy small aerosol cans at most hobby stores. I painted the bellows on an old Nu-View 4x5 maybe fifteen years ago, and it's held up very well to my high-school students' use!