I've done this on a smaller scale. I needed a cheap, but larger than
normal vibratory bowl for polishing jewelry parts. I used a 18" pan for
changing your car oil and an $11.99 walmart fan motor.
The motor was mounted in a wooden ring with 4 springs around it.
Different thicknesses and lengths of springs were tried. the bowl
was attached to the top of the springs on a second wood ring...
cut out with a simple circle cutting router attachment.
I just bolted a piece of brass to the motor shaft to make it wobble
like an SOB....A few experiments with the springs(they're pretty heavy
duty...from ACE hardware) and it worked like an industrial charm.
I still have it. I ran it for DAYS at a time for about 12 years. I was
always prepared to upgrade the motor to a regular 1/2 horse or so,
because I was so reliant on the thing. But it just kept working.
I'll have to get you some pictures. It's a sorry looking thing, but it's
unstoppable. That on an adjusted scale would work great. I wanted
to do the same thing you're talking about to polish flat surfaces, but
I just haven't gotten to it yet. CNC got in the way