This is what you get when you take a soldering iron apart: a handle, a simple resistive heater, and a sheating to protect the heater (or protect other things from it). The tip has been removed in this picture. The nut shown at the far right side of this picture holds the soldering iron tip in by a small lip.
Luckily, the entire thing is hollow! Every single part of it - including the heating element. It's almost as if this thing were made to be modified.