I have a commission to build a custom box for a professional pruner who has a short bed Toyota pickup. Essentially I am building a three sided slatted box with the open end toward the tailgate. I plan on using teak and half-lapping the verticals and the horizontals. Across the top I will set removable Rhino bars for his three orchard ladders. The whole thing has to be easy to disassemble, which isn't a problem. He now wants three stackable box trays to haul specific things and sit on the tailgate at the site. The bottom would be a canvas bag of tools, the middle for the blower and small chain saw, and the top for buckets. I am in Seattle and it rains a lot. Is it possible to build the lower two to be waterproof with the bottom of each box being the lid for the next yet seal?