Double-check those flanges with some good prototype photos. There are good shots on page 111 and the back inside cover of Kratville's book.
The more I think about it, the more I would opt for solid brass on this model. I built my sand dome the way you did, except I used a very thick top to be able to carve a radius between sides and top. Boiler contour is the main problem, but is easily done with a new crosscut file and a sanding drum. Then the attach is by a screw, greatly facilitating the addition, and trimming, of the flange.
Nice work. I wouldn't even attempt doing a flange after the dome is attached - I don't have that kind of patience.
