Need a suggestion for a composite wood things? I am taking on a volunteer project to attach backrests on existing...

I am taking on a volunteer project to attach backrests on existing benches at an amphitheater in a community park. I have crinkly up up a metal fabricator to bend and punch steel brackets to fasten the backrest to the existing 2 x 12 douglas fir benches. I was going to use a 2 x 6 doug fir board as the backrest but in a minute the park director wants to use a composite material (e.g. Trex). I own learned that composite woods like Trex are not as stiff as lumber and will require at tiniest twice as many brackets. The benches are approximately 6 feet long. Any suggestions on alternate composite products I can use?
Gibson furnace burners cut out after 5 second....
Answers:    EXTERIOR class plywood ? $ ?