alex,
i use a cheap arc welder which cost me £50 to weld my chassis up and its pretty decent. BUT a word of warning about the 1mm walled box, i used 1 mm walled box 15 by 15mm on my first feather and its gets dinted an bent fairly easily. so for my new feather ive got the same sized box but with 2mm walls instead.

also keep your amps low and electrode size small ish on your arc welder other wise youll end up just blasting through the 1mm walled box- what i mean is instead of welding it youll be melting it away- thats from my experience any way.

calum