Well, actually do not get real Sellotape, as it is not strong enough! Get the much cheaper polypropylene tape which stretches and cannot be broken by hand. My system of building the back relies heavily on this adhesive tape and it is worth finding the right stuff. Everybody says 'What did the original makers do without sellotape?' but they probably used pins into their moulds with wedges to hold each rib hard up to the next one. No moulds have survived so we don't know. But most makers now use tape.