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Buy a cheap MIG welder (but not a gasless one, they're shit) , an auto-darkening helmet, some welding gauntlets, and have at it.
I would offer to weld it for you, but I think postage could be an issue given the size.
And given the thickness of the steel bevel both sides of the joints and then fill weld the resulting V sections. At least you won't have to worry too much about heat distortion.
What is the DIY approach to welding?
Specifically, I'd like to make a frame from 100x10mm flat steel bar of about 3m x 2m