Reply by Jim Jakosh

I buy a ring of ball bearings and screw them under the top plate. They are pretty cheap now- only a few bucks.
If you made one without it, it seems it will have to be some real slippery material in between with a pivot post in the center with a screw and washer on the bottom so the two plates can’t separate…............Jim

-- Jim Jakosh.....Practical Wood Products...........Learn something new every day!!