I'd go with canvas. You need something that is not going to easily stretch out of shape. We use to use canvas on the back of our big weaving looms because they were always under a lot of stress.. It lasted for years always keeping an accurate distance.

I was lucky, when I got my Blockrockit and Grace frame as a package deal, the leaders were included in the price which was very handy :-). Otherwise I would have made my own too.