Go to any of the fabric sales places, buy a cheap fabric that feels durable, staple and glue in place. Also, sand the roughest spots down first.
For mine, I used the fiber/vinyl stuff you see on concert speakers. While I was at it, put some aluminum ribs on the underside of my base, as it bowed a lot whenever I lifted the mattress.
1987 Starcraft Nova tent trailer, purch. '87, sold 8.14
2013 Crusader 290RLT bought new, 8.14
2001 Ford F-250 7.3
A Loving wife and Mason the always dirty dog!