Boy, I communicate quite frequently with the UK Master Modeler Rob Fern, the fellow that built those KM ML4000's & the Baldwin Sharknose engines in 1/29 scale and he was saying how he has his engines cast in resin..and they don't use either a pressure or vacumn chamber..they just pour it in and poke around with a piece of tubing or a brass rod to release any air that's trapped. His engine shells come out pretty dang good. His shells are thick though..like 1/4" to 3/8" in spots with a few bulkeads inside it to help with shell integrity.
Bad thing about making your own is you know where every little fault is but look over one of the manufacturers products and they're loaded with faults too. That all goes away when our trains are ziping around the tracks!