The first consideration ruled out a traditional dome and pointed toward a roll-off roof design, while the second consideration defined the minimum height and probably conflicted with the first. Nor did I like the prospect of having to crawl around inside an observatory in my declining years. In many cases, amateurs' observatories are constructed mostly with timber frames.
Construction is easy but usually slow with all the necessary cutting and joining.
I found it by looking no further than my garden. The last point was the most significant. All I needed to do was to make a frame to hold the roof together, give it some wheels and a track on which to roll and the observatory was made. Or at least in principal. It would be plenty big enough for two 20cm telescopes, or a very generous size for one larger one.