If I haven't typed it a zillion times, I never typed it!
A solid object blocks high frequency noises.
A low-density object blocks low frequency noise.
I suspect that in your case, you want to isolate the 60Hz hum that the motor makes, and that means mounting the whole she-bang on rubber.
You've probably got high freq noise at the gearbox (if you have one) or around the motor, but that should already be wrapped, so if you need to, wrap it again. Aluminium should do, and is easy (relatively) to work with (you just can't get away from that stuff, can you?).
Your other source would be the bit hitting the workpiece, but here you need to see it, and you may even need to get your hands in there, so I'd recoment and Alu frame, with an acrylic top.
Ok, that's the best of the advice I can give you.