I just made a similar pair of sand box stands, following Mrmb's design concepts. Mine is only 3 boxes high, with 50lbs of sand in each. I used an Ikea birch table top cut into 3" pieces for the legs. And birch cutting boards for the plinth's. They look like maple butcher block, but were more easily available to me and far less expensive.
A few coats of Tung oil on the wood, some gloss black paint on the boxes and a truely nice set of shelves. I did notice however, that the components will benefit from additional isolation under them. I use a combination of rollerblock-like devices and spongy foam tennis balls.
Enjoy,
Bob