Is there a better way of mounting this light, or is my solution the norm?
They work quite neatly with a GoPro adaptor if your helmet has a built-in GoPro mount. Fwiw, the area of contact between your circular vent and the Exposure mount doesn’t look ideal in the pic, but if it feels secure, they work really well, but do sit the light a little high, which I think you can feel with a Diablo in particular.
I’d also try the helmet with the light in the current position before moving it. Bear in mind the Diablo is quite floody, so not as fussy as a Joystick for aim.
The other option, which isn’t going to work with your helmet, is to mount it on a longitudinal vent cushioned on some sort of dense foam pad using zip-ties or Velcro straps. It mounts the light lower, which reduces its leverage and feels better ime, particularly with something slightly heavier like a Diablo. The downside is that you can’t really adjust it on the move.
With the lanyard, if you want lighter/neater you can always butcher the lanyard cord and rig some sort of thin loop through one of the vents. I did something like this with a bar-mounted Joystick on my cross-bike after it bounced out on a rocky descent, zip-tied the plastic connector piece onto the bar so I didn’t have to deal with great swathes of lanyard. At night you tend to notice if you lose a light, during the day, not so much.
I’m sure you could bodge an alternative, lower profile mounting method if you wanted to, one that’s more secure too in terms of branch strike – I’ve had Exposure helmet lights knocked off twice like that – but try the standard light fitting and see how it goes.