I have an old small spot welder which I bought on ebay. it's about 40 years old but works brilliantly .Unfortunately it's very hweavy and when spotting panels under cars it gets a bit tiring.
I also have the remains of an old buzz box welder and i am sure one of these could be used to make a spot welded using heavy cables and a pair of tongs to hold the electrodes.
It would need a contactor to switch the current on and off otherwise the tongs will be live ....
A lot of those homemade spot welders on the web ,using microwave transformers seem a bit small to me and probably only have a short duty cycle. attached are some pics of spot welder extention a mate in England made some years ago. perfect for restoring mercedes floors to the factory appearance and lot easier than endless hours of mig plug welding .
Click image for larger version.  Name: spot1.jpg  Views: 655  Size: 95.0 KB  ID: 583Click image for larger version.  Name: spot two.jpg  Views: 769  Size: 92.1 KB  ID: 584