M8turret
Start by installing three or four bearings which the turret will rest on as it rotates.
They are made from wood screws, where the last 5mm is plain metal and not threaded, cut a short length of metal pipe and fit it on the screw, it should rotate smoothly, screw these into the hull equally spaced around the turret hole, make sure they are the same height and they rotate after you have screwed them up.
I used an old metal geared robot servo to turn the turret, it had no electronics inside and could rotate 360 degrees, powered by an old buggy speed controller to give me rotation in either direction and proportional control.
A model boat tiller has been fitted to the servo shaft which will engage the turret, the top of the servo shaft is above the height of the tiller so that it forms the centre pivot point for the turret and goes into the hole in the turret bracket.
The bracket is made from right angle aluminium about 10mm wide and bent to shape.
I needed to support the action figures in the turret to stop them fowling the turret as it turned, just cut a plastic disc and screw it to the top side of the bracket. I used part of the packaging from a stack/tube of CD-ROMs.
A pair of bolts holding the front of the bracket, behind the gun mantle.
Rear bracket screw, you could have the screw higher up than I did mine and use some stowage around the turret to hide it - that could be a stronger design.