New build with recycled parts on a ridiculous budget 120cm*60cm work area

[Zeeflyboy]

That would no doubt help... maybe flip it around so you can still use your extrusion slots for mounting the rails (assuming that was your plan?).

Best option I think would probably be to use the steel as the mounting face and look into the self levelling epoxy method of getting a flat surface, but that may or may not fit in with your budget option. If not going that route, the question becomes which is the more precise mounting surface in terms of flatness - the steel or the aluminium.