I have often used sheets in the past. I often would use two separate flat sheets, one for the upper part and one for the seat. Lots of tucking until you get a nice fit.
Washed painters drop clotha could be a sturdy fabric too.
Hi Tammy: You sure can. Here are a couple of sites that may help:
This is the closes thing that shows how I did my old one, before we bought a new one. I held up pretty good with 2 babies and a dog for 3 years.