I installed a spa heating element in my quarter barrel instead of a water heater element. I drilled two holes for the element in the bottom of the keg. The beauty of this type of element is it is a weldless type, drill the holes, install and it's done.
You can see a bad picture of it in post #220 of the "Soldering Stainless steel" mega thread in this forum.
Here is a picture of the type of element I used - http://cgi.ebay.com/Hot-Tub-Spa-Heat...item7dbbd02749