Rick's giving you a good way to do it; but one LIFE-THREATENING question hasn't been answered (or even asked) yet - what kind of floor will this be placed on? See, an 8x8 foot "sand box" built from 2x6 will hold almost exactly 1 cubic YARD of sand, which weighs approximately 3000 pounds - divide that by 64 sq. ft. and you'd be adding just under 47 pounds of weight for every square foot of floor surface under the riser.
The majority of "normal" construction is NOT designed for this much EXTRA weight, and may repay your efforts by squashing whoever is below you, or at least adding a few feet of "free" headroom to your space.
So before you do the sandbox thing, we need to know EXACTLY what kind of construction you have; which floor you're on, what it's made of, how thick, the longest unsupported span, etc... Steve
Hey, I thought this was gonna be EASY!??!