We followed our usual practice for baseboards of using 9mm ply for the top and ends, joined by a softwood batten in the inside angle, and cross-braced half way along with a plywood batten. The sides are 4mm ply. Ends and sides are 6" deep, thus creating a light but rigid open box structure.
The tops are trapezoid, making an octagon when joined: we cut seven of the tops to our calculated shape, laid them out edge-to edge and nailed battens across the joins to hold them rigidly, and then cut the eighth top to accurately fill the gap, taking up the cumulative difference between the theoretical and actual sizes. Ends were cut and drilled in pairs to take the bolts, and marked so that pairs were used on adjacent board ends. Thus when joined by the bolts, good alignment would be achieved - in theory!
Boards incorporate fold-down legs: the board with the points from platform to loop has two legs, one at each 'end' [attached to an extra cross-batten of 9mm ply at right angles to the sides, in line with the inner, obtuse angle of the board - see above]; it is set up first. Then the three boards on each side have one leg at the 'outside' end, the inner end 'hanging' on the preceding board. The last board, with the curved diamond of the FY, has no legs - it 'hangs' between both the adjoining boards, and is obviously the last to be put in.
After setting it up for the first time after track laying had been completed, we found that vertical alignment was more difficult, so added small wood blocks to the inside corners to assist this [above], and on the FY boards added some locking fix-it blocks on the top edge to improve alignment there.
The legs would have been too short for comfortable operation had they been left at the length that fits within the baseboard, so extension battens with levelling feet are attached by two screws, one of which is removed to swing the extension upwards to fit inside the board during transit. I will add some photos of all these points for clarification later, when I have taken them!