Hey Mike,
I have been carrying a 19cf pony for about 2 years now. I decided on this size mostly because it is the largest pony I can tuck in next to my tank, comfortably. I had a DIY set-up holding it in place until recently, when I replaced it with the Dive-Rite Remora pony bracket. I am very pleased with this set-up.
I moved up to the AA T2 reg last year, and moved my regs around a bit. My pony reg is an old USD Micra that used to be my air2 in my old USD BC. It was properly maintained and still works great after 13 years. Very lightweight and very small. It already has a yellow case on it.
As I said above... the Remora bracket (quick disconnect) holds the pony bottle nice and tight in next to my primary tank. I agree with several others here, that the reg can be carried around the neck on a tube necklace. I tried that for several dives, and it worked fine. My personal preference on that method has changed.
Because the bracket puts my pony behind my right shoulderblade (I mount it pointing down), I have rigged the bottle with 2 rubber tubes around it as hose holders. I have the hose (I think about 4') laying against the bottle in an "s" shape, under these straps. The hose lays under the straps 4 times, ending with the 2nd stage at the bottom of the rig (next to the 1st stage). In this configuration, the first stage is almost even with my right hand with my arms at my sides (proper diving position). If needed, a simple tug on the 1st stage, pulls the hose from under the straps and the reg is easily deployed. Of course, the valve was opened and checked before the dive, when checking the primary air source.
Three reasons I moved away from the necklace idea... 1) I am already large enough in the water, that streamlining is a real concern for me. I have made several gear changes over the last few years that have made a tremendous difference in my effort to move through the water. For me... less equipment hanging is better. 2) My BC has an Air2 for my secondary, making the pony a true redundant/ bailout bottle. And 3) With the hose and reg tucked in and strapped up to the tank, after the dive, I just disconnect the rig at the bracket, shut the valve, bleed the line, rinse it off, and put it away. Even though it's a 19, the way I rigged it, it's nice and compact. I also put a small pony guage on it, so I don't have to keep disconnecting the reg to check the fill level.
Whichever rig you choose... practice with it! If you haven't used it for awhile, deploy it at the end of a dive weekend and use it to reach your safety stop and then to surface... it pays to practice. And... don't forget to refill it right away!
I hope this helps... you will need to find what works for you.
Scott