I haven't made as much progress on my T-12 #168, holiday and family commitments have taken precedence. However, one can sit at a computer in a lot of places and situations, so I did manage to model and take delivery of the two boiler domes. I modeled them in OpenSCAD, which is a script-driven CAD program. I like using this because one can be very precise about dimensions and placement. For instance, the curved underside to match the boiler was bog-simple to make; just defined a cylinder of the boiler dimensions and subtracted a part of it from the dome bottom. A lot easier than using a fly cutter on a lathe...
Here's what I got from Shapeways, in their "High-Definition Plastic":
Gawd, they're small... :D The Ruler enumerates centimeters.
I probably didn't read the Shapeways docs well enough, I thought I was getting them in an opaque plastic. The translucent plastic makes it hard to regard the details well, but they seem to be nicely defined. The pop valves on the steam dome are well-shaped right to the top stem, and the two little blips on the sand dome are the interface for the sand pipes down to the drivers. The dimensions appear to be within 0.002" of what I defined in the script.
Close inspection reveals faint lines where the plastic was layed, not sure how they'd show up after painting. I'm going to polish the sides as well as I can to approximate the sheet steel wrapping, and maybe let the existing surfaces of the top and base go to approximate the texture of the casting.
~$30US with shipping, glad I got usable parts. I've got other parts to produce similarly and some will probably need multiple iterations, so I just renewed my library card so I can use their 3D printers. I'm going to do a first run there using my cylinders/steam chest/boiler saddle model, see if it'll do definition comparable to Shapeways. I also have some alignment uncertainty with that item, so I don't wan to spend large $$ printing articles I'll probably discard...