Questions about a Homemade Brightstar Big Rock

Hi everyone.I've had some thoughts about making myself some sort of compatible isolation "Table", to sit my HW-19 Turntable upon in my Rack.I assume the Brightstar Big Rocks are pretty much sand filled boxes, correct? While I've seen pictures of these online (but not in person), all they appear to be to me, is a spray painted MDF "Box", with an inner "Plinth" that "rests" inside the sandfilled box lying on top of the sand. Is this correct? Is there anything I'm leaving out? I'd like to perhaps construct one, and any pointers/advice, or even a detailed pic would be deeply appreciated. Thank you, Mark