Or if you really want variable temperature for cheap, attach a cheap soldering iron to a plain old light dimmer knob.
The only thing my 12W (or my 25W one for that matter) wasn't powerful enough to handle that I've come across so far was some bunches of wires soldered together inside a power supply. I got my wife to hold the 25W one on it while I added the 12W one, that worked.
