Turntable.

Here's a couple of pictures of the underside of the turntable to show its workings.
The blue base is a piece of medium dentisty fibre board, which just happened to be painted blue.
The joist has a piece of wood cut to fit the groove and a piece of aluminium bolted to the side for the side stand.
The turntable turns on universal roller bearing houses.

The whole thing pivots on an M16 bolt which is self tapped into the wood floor.