Thanks for the ideas gents. I am afraid that copper forming is out of my league and to chop up waveguides would be unforgiveable.
Jason, the scale of the loco is 10mm to 1 foot (Gauge 1). Oddly the paint (good stuff) will still be there when the solder is flowing freely……that is using standard soft solder.(Bitter experience). I do have some 400 degree stuff which should do the job though. A few rivets will probably assist. The boiler temperature is not the problem…..it is when flames go where they shouldn't or accidental fuel spillage occurs that causes the problem. In a perfect world these things would not occur but not in my world!
A perfectly good alternative and perhaps more realistic size would be 50mm x 15mm …..laid on edge…..but I still cannot locate any. I am now thinking of taking a length of brass channel ….say 15mm x 5mm……then after filing in some "V" sections where the corners are to be, bending it up to form the rectangular shape carcase, perhaps silver soldering the two meeting edges. Then rivetting and soft soldering on the side sheets. I am not very good at that sort of work though, hence taking the lazy route and thinking about using box section.
Mick