Hey Mathew,
Obviously it needs to be permanent, so I would suggest making a transition out of galvanised sheet, attaching to the ring and obs roof.
Cutout 4 of these:
Bend on the light blue lines to match the ring, bend on the dark blue lines to make a flange to match the obs, join the tabs;
Silastic to your hearts content to seal.
Search storv (square to round freeware sheetmetal calculator).
I just used 2300 for the ring, 2300 square for the obs, 75 high and added some flanges.
Make a trial out of cardboard first to get the hang of it.
Neil.
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