Gotta agree with others - have a light that easily comes off for best security. I use a Smart Lunar 35 on the front and a Exposure Flare on the rear. The lights come off but the brackets stay on. The worst that happens is that someone nicks the brackets but this is really vandalism as of no value really. Anyway, one of the reasons I chose these lights is becuse new brackets are easily available separatley.
However, none of this really answers your question so I have two suggestions for you
Firstly you could buy one of these
Defender bike light resists theft and water, looks dangerous
Secondly you could use part of their idea and modify an existing bracket by replacing the screw in the bracket clamp with a wierd/rare scre that requires a specific tool that your average theif wont have.
Perhaps some of the engineers on here could advise ? I've seen triangle type and star type hex bolts