Vette I seen that on the net awhile back. I use to have two old microwaves in storage thinking some day I would figure out a use. Finally thru them away.
Dad bought a commercial unit back in the late '70s. If I recall, it was just under $800. It has two independent leads, one with trigger. Has time set and heat range dials. Works great and we used the crap out of it installing new body panels (qtrs,rockers, etc) but now I can't hardly hold the massive cables up and pressure. The long "C" clamp home built looks more user friendly to me these days.
Main thing to remember when spot welding, all four sides must be as clean as possible to get a good spot weld.