..my first observatory was a paper mache dome (no I'm not joking) that I sealed and painted with white enamel and it was amazingly water-proof. The dome was seated on a a circular rail upturned on wheel supported on the circular besser brink walls. The dome slightly overhung the walls and I ran some flashing down the outside overlapping the walls for about 15cm. Only the wldest storms could push any water in.
cheers,