Even though nobody has had solar panel installs stolen, doesn't mean it won't happen. A lot of tweakers will run off with anything for their next fix, even if it would take a lot of fussing and soldering in order to make it operational again, as they would just tear the wires off the back of the panel in order to get it.
As above, flat roof panels are the best. However, I'd probably use a steel cable and a padlock or two as a way of at least forcing a thief to have to do something destructive in order to get them. This will deter a casual scavenger, and is probably the best one can do for security unless one wanted to fabricate a heavy duty security enclosure.
