Nice ash pan. I sometimes do that with 1/8" square bar.
On the domes - I generally use half-hard brass sheet, and .020 is about the right thickness for getting curves bent perfectly. Use a steel rod. Put the joint on the straight side, back it up with a small piece of brass, and use the solder as body putty.
The flange is the easiest part - I promise. When your dome is done, screw it on, with a piece of thick paper between it and the boiler, and mark the paper appropriately. Transfer that to .020 HH brass, and repeat. You may find you do not need a fillet - it will be pretty realistic without it. But if you do, as above, a small amount of body putty . . .
Don't try to solder the dome to the boiler. That is a loser - you may need to adjust something later. I use 2-56 cap screws, and an allen wrench goes through a small hole in the belly. The screw can actually come through the top of the dome - you file it off before painting.
