I found an excellent article on building an Observatory here I took the design idea of the dome, and modified ot for my needs.
I wanted to keep the dome as low as possible, so I did not create a full half circle for the frame. I drew it out to scale, then cut marine ply to create the frame.
The Frame is supported on Castors to allow rotation. Here it is ready to be lifted into place
The frame was covered in Hardboard, then covered in Fibreglass and painted with Marine Paint. This all took about 3 months.