I have in the distant past had good results with muslim strips painted in Pva or liquid laytex.A strip from one end to the other. It is useful to experiment with a laytex fixer to get a quick drying time. Pva takes much longer. The bellows need to be stretched fully open to apply the strips.Inside. This is very messy and the latex will stick any surface to together if spilt!. Then grease proof paper over the strips and outside so the bellows dont stick together when compressed to allow drying and fixing.When to open is a judgement as you want to be able to unstick any spills before they fully dry.This was my method anyway. Good luck
just a note. I have used latex in costume making and melodeon repairs, its not the best way but does work as a fix but would recomend that after finishing the last coats should be a thin Pva coat, or the Latex will start to stick to itself with age and warmth and bellows may start to stick together. This also gives a good base for painting recomended.