Jason,
I was referring to making only 1 box and elevating it to the correct height by using legs in the 4 corners (and intermediate points along the sides as necessary). You could also make very short stud walls, similar to the walls of a house, and set the 2x6 platform on top of these. This saves on material and gives the box more freedom to expand and contract with changes in humidity during the seasons. Plywood is much more stable and stronger than 2x material.

I started out with nothing & I've still got most of it left
M60 VP160 QS8 EP350
M22 VP100
Algonquins