Thank you.

My thoughts on this: Plywood could become water logged, it is heavy, and would take away tie-downs. If I need to haul smaller items I'll use a Rubbermaid box, or use panniers.

Higher side rails are a good idea for wider loads- I didn't really consider that when building it. My intentions for the shorter side rails were to protect the wheels from cargo slipping into them. They also fold in for slightly larger loads.