Tig Squarewave Inverter Revisted

I have found Iggy's homemade tig welder project and am quite impressed:

I was wondering if anyone can help me out a little. I recently acquired an Lincoln Idealarc 300 amp tig welder which already is an AC/ DC welder with High Freq arc start. The DC side is fine for what I do, but I wanted to improve on the AC side by converting it to a square wave welder. For now all I would like to do is to make it a squarewave w/ variable frequency. Has anyone made an suggesful homemade squarewave AC welder?

I have also discovered this design:

Any help is much appreciated.

Cheers!