@dmk_calgary, this coming week I am taking a 4’ x 8’ x 3/4" sheet of 13-ply Baltic Birch plywood to a cabinet maker and having him rip it into various-sized pieces (22 x 16, 18 x 15, and 16 x 12). I will then make platforms, some comprised of 1/2" granite, a middle layer of ASC WallDamp, and the Baltic Birch ply. Stiffer and harder than maple (for less "flexing"), and hopefully pretty non-resonant. I’m obviously not into "tuning", which I consider the "wrong" path to take.
AVC (Audiophile Vibration Control) offers real nice shelves constructed of two layers of 13-ply Appleply ("pretty" Baltic Birch) with outer layers of cork, but one shelf is priced as much as the cost of a 4’ x 8’ sheet of Baltic Birch plywood and the charge to rip it into pieces, four 18" x 18" granite floor tiles (I have a neighbor with a "wet" saw who offered to cut them to size for me), and the required ASC WallDamp!