So I progressed by adjusting the position of the ring relative to the 2x8s. There is about an inch protruding at the post miters beyond the ring which can be trimmed with the jigsaw. Though I'll wait for the wheels to arrive before making any final decisions. The wheels would seem to fit the center of the track without falling off the 2x8s.
Note that the ring will be raised 195mm or 8" by the inverted wheels. The dome base ring will not be lying on the 2x8s but on the wheels. The 2x8 octagonal 'ring' is only there to sturdily support the inverted wheels at the correct radius. The radius of the octagon posts is too small for mounting the wheels directly.
I'll need to add timber brackets to the octagon posts on the undersides of the 2x8 miters for wheel support. Just as I did to support the main beams/joists for the observatory floor and veranda.
Perhaps I should add some suitable decoration to the bottoms of the brackets on my band saw this time? I simply mitered the lower brackets last time but they didn't look very exciting. Better, though, than a straight 90° cut-off. 2"x4" is undersized while 4"x4" is too heavy looking. Most of my sawn 4"x4" has split badly since purchase. I may look for some planed 3"x4" at the timber yard. Even 0.5m/ 18" adds up to 4 whole meters or 12' with eight lengths.
I wonder whether I should clad the observatory walls before fitting the brackets? If I fit them first I shall have to miter cut the plywood sheathing carefully around all eight brackets. This would demand considerable neatness and accuracy in cutting and might easily lead to leaks.
If I fit the timber brackets afterwards they will need solid packing to fill the empty void where the plywood meets the 4x4 octagon posts t an angle.
I will have to clad the outsides of the posts with shallow isosceles triangles anyway to give the cladding something solid to be fixed against. The back side of the brackets may need to be sawn into a shallow V to match the cladding at the posts. I notched and bolted the posts for the beam brackets but the loads are much lighter for the dome wheels. So bolting the wheel brackets may be enough.
Tuesday: Bought a length of 3x4" planed timber for the brackets. I may need to countersink the coach bolt heads or even for the nuts. I bought a load of 7" climate coach bolts and nuts and still have lots left. My lifetime collection of coach bolts may still come to the rescue if I can find enough of the right length.
Wednesday: Drizzle. Waiting for the wheel courier to arive.