The bottom baffle will be your thickest material, so you don't need the sand as much, and it's kind of hard to get sand there, but it can be done (if you want) .
I considered making a seat, but when I got it to a height that worked well for a seat, with a minimum of three inches space on the bottom (and a cushion on top) the box starts to get rather wide. This means more bracing, more weight and more sand.
Still, I think I may make one like that some day.
And always remember, all of that bracing and sand can get very heavy. That's why mine has wheels.