I used 1X2(fir) around the edges(of 1/4 inch birch ply screw and glue) and the cross supports(also 1X2 fir) sat on top of the 1X2. This also keep equipment elevated off the bottom of the box for air circulation. I curved the top of the box for drainage with a 1 1/2 in overlap. The lid is a lift off lid. Never had a problem with water getting into the box. With the curved lid the box was strong enough to support my 245 pounds. I coated the inside with polyester resin to keep any moisture out of the wood. Total coast to build about 145 dollars.