If you can post a picture of your helmet showing how the vents are arranged and describe which vents you want to use that may make it easier for people to give you good recommendations.

Also, not quite as important, but may have an effect on mounting options, is your light a P2D or an L2D? I don't think there is a Fenix P2LD (sure would make things confusing if there were).

Here's how I attach my Fenix P3D to my helmet with a Livestrong wristband and an extra rubber spacer from a blinky. This may or may not work for you depending on your helmet vent setup. You could substitute an O-ring or some rubber bands if you don't have a Livestrong band laying around.

I was going to include some still photos with better lighting/detail but apparently Photobucket is temporarily down for maintenance. I can post some still photos later if you think that would help.