After some experimentation, the dome construction method that works best for me has been the fast machee and spackle method (I'm fibreglass averse, but also like to do things myself). Bill's dome was made of newspaper, and I didn't get a very good bottom edge. The new dome is still not perfect, but much better than Bill's.
Bill's dome plug was a 1/2" too small, so I made another one. this time I drew a circle on the base board of the right diameter and made sure that the template stayed on the line as it rotated.
When spackling the dome, I went around the lower 3" of the dome with a straight edge, which helped keep the surface nice and smooth.
After sanding I sprayed it with a heavy wall primer (Kilz brand in the US) and then used automotive primer and paint over that.
In case anyone's wondering about the strength of a fast machee dome. Here's a picture of me standing on one!
With Bill, I used the classic centre pivot method to mount the dome. With Dirk I decided to go for a pivotless dome, to give me a little more headroom, as I'm a little over 6' tall, and Bill is cramped inside for me.
I stacked up two wooden rings, cut on a router so they are perfectly circular.