One of my favorite scratch building projects was building a turntable. Mine was planned for and built with the initial layout in 1978 or so in Jamaica and cost less than $5. It used a spare clothes dryer belt for rotation and uses the "ole eye ball" to line up the tracks. A 6 inch Lazy Ssusan bearing was used to mount the pulley (made from the cutout for the TT) under the floor of the TT. Power is fed through a piece of 16 ga twin strand wire and must be untwisted once in awhile if one direction is used much more than the other. Still works great after 4 or 5 houses and moves.
Picture above: Notice the pigeons above the loco and poop below!
The picture above shows the hand crank used to turn and spot the turntable. A 10 position rotary switch determines which track will be powered and is in series with a momentary push button switch which powers that track and the TT track for operation.
Lets see pictures of your turntables, about the best operating addition one can add to any layout and does not have cost an arm and leg to provide lots of fun.
Charlie