Thanks for the info Max! Glad I can move forward on that.
I've done some work on the boiler today. I'm no master craftsman, so I prefer to take the easy route when possible, even if it takes longer in the end due to mulling designs over in my mind. I'm not very good as soldering and I couldn't find any styrene rod large enough, so I've decided to take a hybrid approach to the boiler.
I first made quickie castings of a LL berk boiler to use it as a base. The biggest reason behind that is the snap fit. I then cut brass tubing for the sections.
Here's a test fit after a little bit of hacking the shell and milling a slot in the brass tubing. You can see I trimmed back the inner brass tube too. Also it should slide back a little bit further but that lone pin on the mechanism needs to be milled away first.
The smoke box will be a separate piece that slides into the boiler.