There is a huge difference in the quality of pony mounts.
Most SCUBA equipment is good. Some of it is excellent. This is the one used by rescue units and the military because it remains just as quiet and perfect a fit over the years as it was when it was brand new. It is designed to accept long term wear without losing its perfect alignment.
In gear to mount pony bottles or battery packs, there are many mediocre ones, some bad ones and only one excellent one. It's called the SR-71 Mount. It's made in the USA by Patco Inc. a high quality designer and manufacturer of high tech diving equipement. I'm going to take the space to explain it to you because it is in the same price range as most of the others but the quality is only with this one.
This picture shows a generic designed mount. It is the right idea, but it lacks the quality of design and therefore creates problems. It consists of two pieces. The top piece (female) is attached to your pony bottle or your battery case with two stainless steel bands. The bottom piece (male) is attached to your pimary scuba cylinder with two stainless steel bands. If youi want to switch you pony bottle or battery case to other primay scuba cylinders, you would have additional of the bottom pieces already attached to other scuba tanks. When you put another tank in your BCD, you would simply release the female piece from the male and place it on the other cylinder.
What's wrong with this system?
The SR-71 looks very similar but it is made in an American machine shop by machinists, not in a factory in the Orient by someone who was just taught to work some simple machines. If you don't look closely, you won't notice the differences but they are actually extremely important. There are the same two pieces, a male and a female piece. But the similarity ends there. If you look at the male piece, you will see that the male pin is tapered in two places. The tip is tapered to make it simple to insert into the female tube. And, the base is tapered as well. If you were to look up into the female tube, you would see that the entrance is tapered to match the base of the male pin. That cone shaped orifice not only makes the mounting extremely easy, but it insures that the base of the male pin will seat snugly into the female tube.
What's right with this system?
Here is a detailed picture of the femaie mount so you can see the tapered opening and the curved fitted bracket. The automatic locking latch is spring loaded so it can be placed on the male pin using only one hand.