Jon,
I don't think they used a turret ring, but a series of rollers similar to the setup on the US M3 series Light Tanks and M8 armoured car. Go to the earlier thread
Vickers Carden Loyd Commercial Light Tanks and lookup the link to a download of AFV Weapons Profile No 5 "Light Tanks Marks I - VI", that booklet has a lot of info.
Like Lynn said, it will take a considerable reconstruction to turn a T16 into a light tank. They had the engine to the right, driver to the left, which is not easy to reconstruct. Ideally, you would need a differential offset to the right. An alternative is to move the engine to the rear and connect it to the gearbox/differential with a prop shaft (just like in a Stuart and Sherman tank).
Lots to ponder about, am looking forward to hear how you plan to tackle it.
H.