Plastic bedliner 'slots'

2 inch thick: 2 x 6's or 2x8 lumber would be good if you only plan to use them as cross members and don't plan to attach a plywood deck. The 2'' thickness would be great for strength and less sag in the middle.

Composite Deck Material: This material is notorious for sag in the middle and structural weakness. And it's very heavy.

1 inch thick: Wood decking material called 5/4 (five quarter) is slightly thicker than dimensional lumber like 1 x 4's or 1 x 6's. And it is substantially stronger (resists sag in the middle). Plus it has rounded sides making it smooth and nice.

I plan to use the 5/4 material for cross members.

 

Let me know how it works out.