To cut down on any distortion, rather than go all out and nuts with the welding, I clamped everything in place and applied small spot welds to hold it all in place then tried not to let it get too hot for too long while keeping an eye on it with squares and a straight edge. I'm also using steel with a 4mm wall and its going fine. I'm only using an el cheepo arc welder with 2.5mm rods. I tried using 3mm rods at first but couldn't strike an arc without blowing fuses. The day the sun was out is a long gone memory.. ..sigh.