Here is a simple way to do foam/fiberglass. If you don't care about what the finish looks like, you can skip the bondo, and just paint.
If you are going to do sheet metal over a steel substrate, you may want to look into how ambulances are built. The big box that goes on the back is aluminum welded into a frame, and the skin is aluminum sheet that is adheared with double sided tape (industrial grade no doubt). They spray foam in between the aluminum structure, cut off the excess and double sided tape the interior into place and use vynal as a floor. There are lots of reasons for using tape over rivits/screws. It is less expensive, seals to the outside better (particularly important in an ambulance), holds better, and offers an additional layer of insulation for both heating/cooling and sound deadening.