These are known by a few different names (Billy, Thermette, BackCountry Boiler, storm kettle, volcano kettle, Kelly Kettle, or Bush kettle) depending on your locale. They all work on basically the same principle of a water jacket around a wood fire to boil the water.
The commercially available varieties are quite expensive however, and when I spotted an old broken electric jug in my favorite local dumpster, I had to have a go at making one.
The process I used involves fabricating a cone out of the same material as the donor jug, and welding it in place, making a donut of sorts.
I've seen copper, aluminum, and stainless wood-fired kettles, this one is stainless steel, and to my knowledge is the first of its kind with transparent viewing windows in the sides.

Materials:

donor electric or stove top jug
donor thermos (optional)
sheet metal the same as your jug
fork (optional)

Tools:

welding setup
Dremel tool
drill & bits
angle grinder

Step 1: Gut the Jug

The first thing I had to do was get rid of all the extra bits I didn't need. Fortunately I had the 3 point driver bit I needed for the tamperproof screws. With the bottom & the handle off, all I had to do away with was the heater element. Ye olde multipurpose chisel made short work of that, a few good whacks and it popped right off.

Step 2: Cut Some Circles

For this part I cut a piece out of the bottom of the jug that was just big enough to fit over the top of the jug. The size I needed was about 3 3/4" in diameter, but it will depend on the jug you use. If the opening at the top of your jug is small enough, you won't need to save the center cutout, I wanted my top hole about 2 1/2" diameter so I had to weld the bottom to the top in order to make up the difference. I also domed the circle for the top a little by beating it into a piece of soft pine with a ball-peen hammer.

Step 3: The Cone of Shame

To get the inner cone I first made a template out of cereal box cardboard, slit it from top to bottom, and traced it onto my sheet metal. The stainless was thin enough so that once it was cut out, I was able to roll it by hand into the shape I needed. When the cone was formed, I welded the seam and cleaned it up.
I'll bet you could salvage a decent one of these from either the inside or outside of an old thermos if you had one.

Step 4: Weld It Up

Once I had the cone formed, I welded the top on, traced the inner circle onto it, and cut that out. Now the holes in both top & bottom exactly fit the cone. I carved an insignia on the cone with my Dremel tool so that it could be seen through the side window, double checked that all the seams were tight, and welded the top and bottom circles. The Dremel also came in really handy for cleaning the welds I couldn't reach with the angle grinder, like inside the cone.
I probably could have gotten away with using the original jug handle, but opted instead to use an old stainless fork I found in the A/C ductwork of my VW. I bent it to the shape I wanted, it really wasn't keen to get welded to the jug, but I got there in the end, albeit a little ugly.
Note: you will probably find, as I did, that there are some holes in the top of the jug near the handle. Don't weld these up or you won't be able to pour the water out of your jug, and you'll have to drill them out again and feel a bit like an idiot, as I did.

Step 5: Fire It Up

Once your new kettle is complete, fill it up with water, prop it up on something fire proof and light a fire under it. You can even make a fancy little stand for it if you like. I was surprised at how quickly the water boils once the fire is going well.
Unfortunately the graduations on the windows are no longer accurate, but the jug can still hold about 1.25 liters of water. Also the fork handle stays cool enough to hold even after the jug has been boiling for some time.
I'm sure this will be a welcome addition to any future car camping trips in my VW camper!