If you insist on using a soldering iron, cover it with a Pyrex test tube, as is the standard design in commercial ones. It strikes me that the wooden housing of the iron seen in commercial peices resembles an electric pencil sharpener with the guts taken out. This would be a good way to mount it. Also, a variable resistor can be used to make a temperature dial. I'm not sure of the exact mechanics of this, but it is useful to be able to control the heat. I suggest low heat for most of the process, but turn it up so the bud burns right at the end (once you think you've used most of the THC; this will ensure you get every last bit). Vaporizing can be fun, but don't let it get in the way of the true pot experience; spliffs are still the ultimate smoke!