P.T. Pine and...
a fabric that won't rot like the stuff that you can buy in the trailer secton of marine stores or I/O carpeting, and don't forget the tacks should be non-rusting like copper or aluminum, and the bolts noncorrosive and countersunk if not side-mounted. On my dinghy trailer, the bunks were straight and the bottom round, so the flexing broke a moulded joint. Make sure your hull is happy with the fit, esp under road conditions.