Just curious here, I assume you dive a single tank? Why do you need a 'pony bottle' ? Your backup gas is in your buddy's cylinder if you have do your gas management properly. If you really need more gas you should be diving doubles. I used a pony for a short while until I realized that it is not a solution to any problem and creates more problems than it solves. A pony makes your whole rig unbalanced. If anything, carry an extra cylinder as a stage tank clipped to the left hip and shoulder D-rings so you at least have proper access to the tank. Proper gas management eliminates the need for a 'pony' bottle. Just an idea.