If you use anything fabric just check it every so often for stretch.
I know one time for a precise dance troupe I worked with we were looking at creating one out of rolled plastic or vinyl - sort of like a small strip of marley itself. I don't think anyone ever accomplished that project. We did have one out of jute that stretched, and when I left we were using a big yellow webbing of sorts. For our purposes you also had to watch how straight and centered the numbers were on the line, too. For the actual floor we used mailbox vinyl numbers, but you had to make sure all numbers and their backgrounds were the same widths (if you had a skinny one with a fat zero, the center of "10" would be judged differently by different dancers).