Try the plastic "wood". It cuts and fastens like wood using the same tools. I lasts a long long time. Buy some extra for those annoying things around the house you have to keep replacing.

There is a 2x6 made of recycled tires for docks and such. There is also a "plywood" made of what looks like recycled milk jugs called starboard. You can get the Starboard at West Marine and for 2x6 check lumber yards that supply material for docks.

Anouther alternative is the mast holder Hobie sells for the Wave. No moving parts and you bungee it to the rear cross beam. If it fits it's great