I have made this same setup in 1(good for early force carb sampling), 2 and 3 liter bottles, I still need to pickup a 6 liter bottle. I used a PSI gauge was because I would not be able to feel the pressure in the bottle in the cooler, but you can tell by the pour when you need a "SHORT" blast, they maybe small but they have all the pressure of the big boys and no regulator. I found the gauge is easily ruined by the pressure.
If you choose to use one of these injectors, be warned they are not a toy and could cause serious harm if someone was to over pressurize.
Here is another example for you, I finally got around to putting some pics accessible to post.