I though of using 5 utility poles, and building a platform for a dome on top. 4 very well braced poles for the corners and the 5th pole unattached which would come right through the centre of the dome floor untouching the structure for a pedestal..the poles could be closed in at your leisure, giving you a down-stairs as well. You may even find used telephone poles at the power company yard which may be less expensive.
(This is just an idea)