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Gatorbak is another option. You use treated wood like usual, but instead of using carpet you install the Gatobak plastic covers instead.

http://gatorbak.com/

Gatorbak is supposed to be a good...

You can use high density plastic (HDPE) 2x4s and 2x6s - it costs more, but also requires more supports as it can't span the same distance as wood. Some companies add aluminum channel to increase...