The sand mould worked! I have mostly-straight rails of the length I wanted, and they didn’t explode, and they seem to be pretty stable!
Now I just need to do that a few hundred more times.
Plus they take a loooooooong time to cool.
So I’m going to make more frames, to make more rails, and see where I get with that. It’ll give me something to do in the evenings that isn’t sewing chicken skins together to make leather.