Good point John, hadn't thought about "top heavy". I've sent an email to Dave @D.C. rail, waiting for a response, he's only about 20 miles away.
Laz1957, thanks for the video.
Maybe use some “Great Stuff” or equivalent to save weight when building up the coal piles on the plywood base. One can would make many coal piles. Don’t forget to remove the base from the car when applying the foam.