Trying to find the mass of the roundabout

 I was back on the roundabout. I timed myself as 7 seconds per rotation near the centre (say 0.5m from axis) and 8 seconds when I was about 1.5m away.

The movement should involve conservation of angular momentum. I have added my point moment of inertia to that of the roundabout, which I have taken to ne a disc of radius 1.5m. The result is a mass of 730kg for the roundabout. It might be a bit high but is not totally unrealistic.