We've made a lot of progress since my last posts:
Stage: I sucessfully filled the stage with 2,000lbs of sand plus a about 30 cheap concrete patio blocks (didn't have time to dry out more sand). Drying sand took much longer than I thought. In the Home Theater Room with a dehumidifier and a fan took about 12 hours/bag:
I utilized my two patios to dry about 8 -12 bags/sunney day to finish up:
I used crossmembers in the end 2 cavities to make sure the weight of the sand kept the stage down. The pipe in the photo is the drain from our wash machine. It used to make a loud noise and now under 6 inches of sand can't hear a thing:
Here's a shot of the concrete blocks. I used them to take up volume. The rest on sand, are covered by sand and do not touch the edges of the cavities:
I completed the stage by filling each cavity slightly over the top, folding the plastic over the top of the sand and screwing the top on. Carpet came on May 30th, and I'm glad to be able to close this chapter. It turned out great.