I was reading something over on HF about this, and one member suggested mounting directly under the mainshaft. Seen the video he did and it seemed to work for him.
I have the keychain camera as well and have yet to try it, but I have three different places I'm going to try as soon as I get my butt out to fly. I have a mount on top of the canopy, between the skids under the battery tray and directly under the mainshaft. So we will see what works best. I think out on the tailboom is just amplifying any vibration there is no matter how small.