Sorry Jorge, nobody seems to respond to your question.
Personally I've repaired some of my 8x10 and 5x7 Sinar bellows with quite thin black tape,
www.stokvistapes.com Works fine, as far as I can see, and wasn't very difficult. I heavily stretched out the bellows, looked more like a cone with sharp edges (don't know the proper name for this geometrical figure!), put the tape the whole way over the edge. Only problem was to get a good ending, easy to make a good start. I suggest using one piece of tape from the front and one from the rear side and make a proper meeting half way. You should try to get one hand inside the bellows meeting the other outside - not bad, I think, if you have more than two hands! Ask some friend to assist holding the tape. Properly done it should work quite well - and not necessarily ugly, quite the opposite!
//Bertil
