I decided not to turn the domes' "cosmetic" top covers from solid (I was out of large diameter brass anyway), so instead I used the age-old technique of "spinning".
Starting with a 1 mm (0.04") thick round piece of soft annealed brass plate, and a former I had turned of suitable scrap metal, I set them up in the lathe chuck...
... and started to form the plate with a very simple tool I had made: just a roller (a 20 mm / 3/4" dia. ball bearing) attached to a piece of steel rod. Here the brass is already being pressed towards the former, using the roller tool in the lathe toolholder.
Applying pressure while moving the roller all along the edge, with the roller always perpendicular to the surface, the brass soon assumes the shape of the former.
The process is very easy - in just a few minutes, I had formed the brass into a cup, and cut off the slightly ragged outer edge with a parting tool.
Here is the finished dome top, and the former I used. It is very important to use dead soft, annealed brass (or copper) for spinning. Hard brass will crack!
The dome top is now soft-soldered to the dome tube and the assembly has been sanded smooth and painted with spray primer.
The hole in the top is for a stud screwed into one of the boiler fittings - an acorn nut will be used to keep the dome in place.