I used an Atlas "N" scale turntable.  Wood blocks are glued to the outer ring and a new floor of wood was glued down.

The very center is open, about 1.5 " circular. A wood support pillar under the bridge is glued to the original floor and jumper wires are soldered from the "N" rails up to the rail on the bridge. On this one no detail of the center race was made, can't see it well anyway. Of course an "HO" turntable can be used, but I had little space.