Garmin mounts and Gopro mounts have become de-facto standards; you can find lots of mounts, adaptors, and accessories (including custom 3D printed mounts that you'll see referenced on this very forum). I've got a tiny adaptor for my Lezyne headlight that lets me mount it to a Gopro mount, for example. Garmin is good for easy-on/easy-off and lighter gadgets. Gopro is good for semi-permanent attachment and heavier gadgets.