When I got my L2D, I tried the TwoFish bikeblock but wasn't happy -- it didn't fit my helmet well and slipped around while riding. So I played around and came up with the mount below which works much better for me.

I used the large O-ring that came with my Dinnotte headlight, plus a plastic wine cork and a small O-ring from the hardware store. The big O-ring goes under a bar on the helmet and wraps around the front and back of the L2D to hold it on. The cork raises the front, and the small O-ring slides back and forth on the rear of the L2D to adjust the angle.

It holds its angle very well and is surprisingly secure. Last night, I didn't duck enough coming into the garage and banged it against the garage door as I rode in. The big O-ring sprung the light right back into place no problem.

The helmet I'm using right now is my commuting/winter training helmet, a Bell Metro. It'd take a little more surgery on the cork to get it to rest well on my Pneumo.