I think just using screws by themselves could be a real problem. In the case where you are using a Brush Finished 9" Base where  Philips Screws, screw down into the plywood, it wouldn't hold well.

  In my case, I'm using 3/4" and 1/2" plywood sandwich where I can use a carriage bold up through the plywood and a nut on the topside of my economy mount/pedestal.

Bruce

Try using T-nuts. Search on the internet if you don't know what they are. Use locktite or a similar threadlocking compound to keep the bolts from loosening.

Do you have access to the underside of the bench? This will limit your options if you don't