i am doing an oblong firehole for my current loco boiler, a 5"g LBSCR terrier. this is true to prototype on my loco. i will turn the ends of copper 10swg firehole tube in the lathe then anneal and form to the size and shape i want.
have a look at Don Young's NEWPORT design in 7.25"g which has the same type of firehole shape.
cheers,
julian