I'm sure I'm not the first to do this but it's first time I've tried and thought I'd share..
I was messing with light to adjust it one day and found a good spot - mounted upside down. Added bonus that it's less conspicuous this way too as it no longer protrudes (sometimes, I take a chance and leave light on the mount after I park and I've had a couple stolen). Me thinking this is less likely to walk away. Plus, it's not as natural to dismount it this way so I'm thinking the type of person that steals bike lights might give up pretty quickly if they can't get it off in a hurry.