Now that the decision to work on the ground has finally been made, I might as well treat the job seriously. So I have ordered eight [fairly] inexpensive trailer/caravan stabilizers. These come in sets of four so I shall have eight to play with. Each stand can rest on a 8" square of 2" thick timber as a load spreader on the increasingly muddy lawn.
These pyramid stands have an aluminium base of about 160x160x220mm [6.3" x 6.3" x 8.75"] and height adjustable from about 270-430mm [10" -17"] in Olde Money. Claimed load capacity varies between 750kg and 1ton each, depending on the supplier. So eight will be more than adequate for supporting a 10' plywood dome evenly at a comfortable working height.
I can add scraps of ply to the top plates to spread the load into the base ring more evenly. These stands will also lift the base ring above the inevitable winter mud. Perhaps a car trailer of gravel over the entire working area would be sensible? It has certainly made the octagon area a pleasant and practical walking surface. The stand supplier has promised a Monday delivery. Though more likely a Monday dispatch.
A clear and bright afternoon gave me a chance to retrieve the now much heavier, 32mm / 1.3" thick, double base ring. I used the boat winch to lower the ring back down to the ground after it had been exposed to heavy rain. It was largely my inability to properly and safely protect the ring from the foul weather which had tipped me into working on the ground.
Sunday: Found a local stockist of the stands. They really are quite lightweight but rated more than adequate to the task. I was tempted to bring four more home but decided to see if eight were enough for total stability first. The package has been dispatched [on a Sunday] so I'm now at the mercy of the post office.
Monday: I was bored waiting for the post. So I rebuilt the dome skeleton with the ring resting on wooden blocks. Then covered it with lightweight tarpaulins just as the light went. It will all have to come apart again to go on the stands. If/when they arrive.