As far as scratchbuilding one. I already have to for an MDC narrow guage 2-8-0. The bunker would be easy, except for maybe rivets (they would be rivited right? or welded) and the hatch but I figure companies like persicion scale or cal scale would have one.
heck, its your model, you can have whatever you like?
who is to say it wasn't welded up at some later date?
Rivets are easy....suggestion being, an overlay of thin plastic sheet, embossed from the blind side using a blunt pointy thing? (do it on a 'soft' or 'forgiving' surface...ie not the dining room table!)
Welds or rivets will largely depend on your actual era modelled?
I may be wrong, but didn't arc welding really only come to the fore around WW2?
for a 'filler' cap (it doesn't HAVE to work , honest)....a short bit of 'tube'..with a circle of..whatever, glued on top, 'top-hat' fashion?
Then perhaps a bit of bent wire for a 'handle' to undo it with?
MAybe a coil of hose? (solder wire?)
what about a (scrap) tank dome/filler from an N gauge tank car?
Unless you follow rigidly a specific prototype..then you're stuck with the expensive route?