A couple of months ago I bough a DIY spotwelder from keenlab.de.
After creating a suitable power supply I have to say: This is the best product I have ever bought.
You can set the amount of energy (in Joule) you want to put into the weld. It has an automatic mode and
a foot-pedal so welding is quick.
The only difficulty is the power-supply, I use a 2S 20Ah high C-rate battery to power it. You could use a transformer/rectifier or
even a supercapacitor bank to do the same.
Here is a link to my IMGur album: LINK