Initially I questioned the longevity of painted plywood, but it keeps cost down, allows for easy design and creation, and makes modifications simple.
But on second thought, about 10 years ago I made a much simpler box for a Harbor Freight trailer for hauling construction tools and materials for side jobs. I did 1/2" pressure treated plywood on the roof, 3/4" OSB on the floor, and regular 3/8" plywood on the sides. It has long since been retired as a work trailer and has been crammed behind the garage, where it serves to keeps the rain off my push mower. It doesn't look nice, but nothing has rotted away. In fact, the trailer frame is in worse shape than the wood box on top of it.