Cool Instructable from user hpstoutharrow. Of course these propane bottles are not meant to be opened, so you have to be careful to follow a sensible procedure, but it seems to me like hpstoutharrow has done a good job in providing that. Commenters there seem to agree.

Once any residual flammable gas is safely vented, both valves are removed by drilling, and the bottom of the bottle is cut out to insert a 20 oz plastic beverage bottle, which has been shrunk slightly by exposure to boiling water. The neck of the soda bottle protrudes where the tank’s center valve was, and is secured there with an o-ring. Spray-in foam insulation holds the liner in place from the sides and bottom, and insulates its contents from the metal bottle. Finally, the bottom of the tank is reattached for appearance’s sake. [via Hack a Day]