Get a piece of Baltic Birch plywood (the stuff is pretty stiff), and if you want to go nuts glue a piece of MDF onto it. Even better, put a layer of ASC WALL damp between them (the resulting shelf will be very non-resonant). Add a set of bolt-on legs of whatever height you desire. Try different heights to find the optimum woofer location.
If you want to isolate the platform from the floor (and therefore the sub along with it), put a spring under each leg, or between the sub and the platform. If cost is no object, consider the Townshend Audio Seismic Pod, a superior isolation product employing springs in a patented design.