For rhat size try cutting 30 x 30 hollow section down. It will stay reasonably straight if you anneal it first. It will go banana shaped if you don't.

Martin

Metals4u have 30 x 30 x 3 mild steel you could cut down or 30 x 15 x 4 stainless.

Stuart

I clamped the two pieces down onto some flat plate and ran a few short tacks on one side, turned it over and did the opposite side and then kept on doing a bit each side until it was all done. Idea was to try and balance out the previous weld so it did not all get pulled one way which you would get if you just ran a weld all down one side in one go. Milled off my poor welding after to just leave weld in the Vee preps.

More details and photos part way down this post

You could try here **LINK** this is now sold by Steam Workshop I believe, but was designed for underframes so may be suitable.

I had a look at that, scaling from a drawing on the interweb, the channel seems to be 18" deep, whereas the biggest Steam Workshop sell is 12" (scaled down to 5"g size). However with the construction of the loco, you wouldn't necessarily notice if it was folded from sheet, so a trip to the Middleton Railway seems to be in oredr, they've got one which I can measure up