When driving on rougher roads I can hear a little tiny bit of a vibration in the back. It appears to be the bottle vibrating within the bracket, and not the bracket itself. I'm going to try and add some small pieces of rubber to the inside of the metal bottle strap that runs across the middle of the bracket to see if that helps...
It appears that vibrations I spoke of earlier (from bumps, rough roads, etc) were travelling up to the bottle/bracket and on to the weakest mounting point, which was of course, the plastic sidewall behind the top of the bracket. I was able to recreate the vibrations by pushing or pulling against the powertank handle and moving my hand real quick.
I think the vibrations were travelling to a spot in the plastic that was closer to the rear hatch, because I was able to muffle the vibrations if I put pressure against that spot with my hand.
I went to a local upholstery shop and had them cut some pieces of grey colored foam (almost match the color of the plastic) to play around with. I cut a pieced to fill in the cubby hole behind the bracket. I thought that would help, but it really didn't seem to do a whole lot.
What finally stopped the vibrations was this...
Yup, a big piece of foam wedged between the handle and the bracket. Luckily, I don't think it's too much of an eye sore.
Regardless of the vibrations, this thing is still pretty sturdy. I've had it over some rough terrain and jarred it pretty hard. The bracket hasn't budged.
That being said, however, I would have my doubts about mounting a 10# tank in this manner. I'm not even sure it would fit in this spot. Plus, the 10# bottle will add 7 lbs. in empty weight, and an additional 5 lbs. in C02 when full (that's not even taking into account the additional weight from a larger bracket). I think it needs to be mounted to something more solid than the plastic. But, there are some creative people on this board and I'm sure someone can figure it out.