The risers were made using the same router rig technique as the rings but sandwiched twin ⅝ inch ply sections were used instead, to limit the total weight and because it was considered thick enough to allow for overlap of the adjacent sheets and their fixing down.
Wood screws were also used to secure and bind the two leaves of the risers together.
These were then bolted by angle brackets to the top side of the base ring at the bottom and and to the side of the arched beams on top reaching to the top of the shorter ⅝ inch thick arch on the outside of the main arched beams. The angle of the cut at the top of the risers was arrived at by using cardboard templates and that time honored method…trial-and-error.
… and it starts to take shape
When all the ribs were installed and their brackets firmly screwed in place the whole structure already was quite solid and reassuringly whirred easily on the castors.