If you're looking to do a onboard/portable welder on the cheap. I found this a few years ago.

http://www.huv.com/jon/jeep/Welder/on-board-welder.html

It's designed to be ran parallel to your regular alternator. A small gas motor and a cage would make a nice portable rig.

 

Two batteries, jumper cables, and a welding rod. It's cheap, it works, and takes up next to no space. Granted there is no control, but in a pinch you can get couple pieces of metal stuck together.

Wouldn't recommend it other than for emergencies.

I do like the Ready Welder.