First of all i do not claim to have thought this one up. I kicked myself when i learned about it though. Such is life.
If you have an old microwave transformer you are half way there.
Here is my first video (lots more to come as i work)
http://www.youtube.com/watch?v=yDn1CTxBeMc
I also have a blog about it. Alot of reading but it should give a general idea as how i went about this.
http://diyspotwelder.blogspot.com/
I'm not going to make a step by step here, Simply passing on some information for anyone who is interested and has the means to make one.