I think on a wagon, the roof rack is a great alternative for the spare. It may (will) reduce your fuel economy but you could create a fairing in front of it to help some or some sort of aerodynamic "box". The spare tire / oil drum tank is definitely the way I'm going to go once I get the nerve to do my conversion.

Of course, this will take the place of the spare tire CNG tank I was considering but I like the WVO idea better anyway. Not sure I want a CNG tank inside my passenger compartment, anyway!