Hardy planks would actually work pretty well. They are certainly not light, but they would last forever. Most builders hate it because they go through blades so fast on the cutting, but a bit of paint and a clear coat and you have a very useable bed.
Trex is another composite that might work as well. It's like 2x4's made out of recycled product. It's integral colored so scratching it up wouldn't be as big an issue.
There are certainly a lot of options.
