slaw’s point is well taken. While a stiff shelf is better than a weak one, neither provide isolation to any appreciable degree. Baltic Birch is not only stiffer than MDF, it is lower in mass, so it exhibits a higher-frequency resonance, and absorbs, stores, and releases less energy. The problem with Sorbothane-type rubber products is that they isolate down to a corner frequency of around only 10Hz, below which their isolation properties roll off, becoming a low-pass filter (very low frequencies pass right through). A turntable benefits from isolation to as low a frequency as possible---10Hz is not nearly enough.
The best turntable isolation is that provided by the electron microscope platforms made by a number of companies, but they aren’t cheap---over two grand. Roller bearings (Symposium Acoustics, Ingress Engineering, FIM) provide good lateral isolation, but couple in the vertical plane. The best lower-cost alternative I am aware of is the Townshend Audio Seismic Pod, at around $100 a piece. Put a set of those under your turntable shelf (and/or speakers, CD player, and electronics) for isolation to a pretty-low frequency---2-3Hz. Watch the Townshend video demonstration of the Pod on You Tube.