I started with a Harbor Freight trailer. The bed is 4×8. The birch plywood bottom was coated with trailer waterproofing paint. I’m crossing my fingers that it will hold up for years. The shims I’m painting will support the base since the HF crossbeams are about 1/8″ below the side rails.
To get the exact spacing, the floor beams were screwed and glued directly on top of the trailer beams.