What I did was drill four holes in the aluminum patio roof, then run four long screws up through the roof and secured them with a nut. Then I used washers to try to level things out, then dropped the gauge down over the screws. Then I measured with a bubble level, removed the gauge, added/removed some washers, dropped the gauge back onto the screws, and measured again. Repeat until level. Then I used more nuts to secure the gauge in place.
Then it was just a matter of screwing the solar panels to the side of the patio and aligning it to get the best reception.
I've now got a full set of weather instruments on the roof, just waiting for the next big storm. Too bad this is the dry season in San Diego and it probably won't rain for another 4 months. :P