Do you have logistics posts in your trailer? If so, hit up a moving supply for some load bars and then a lumber yard for some plywood. 1/2" could get you by but I always liked something thicker. And no particle board.
Once, LONG ago, I had a nursery load needing double stacking, had forgotten my decking , so made up some using a pallet on each side of trailer against the wall, and a pallet in the middle of trailer, all standing on edge.Placed a 1/2 inch sheet of plywood on top of the stood up pallets, and nailed to pallet. , Worked just fine, just a pain in the butt.