What about making or adapting the same thing from plastic, possibly with slightly different shapes but with mating threads? Then you’d have a small metal vessel with plastic lid, and a small plastic vessel with a metal lid. Break the visual signature of “something dangerous” and the probability of passing the cursory check shoots up.
Likely the entire thing could be made out of a hard enough plastic or ceramic, besides the spike in the lid that punctures the top of the n2o canister.
Essentially these work by the lid screwing down on the canister and a spike in the lid piercing the top of the canister releasing the gas which goes through the hole in the lid and into a balloon you’ve put over the lid piece.
Now days they sell co2 bike pumps that take the same size cartridges and allow you to control the flow.
Looking back in life, I’m pretty sure the younger me’s main goal in life was just trying to make sure that the older me would have an interesting auto-biography! 
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They get very cold with use. The aluminum ones tend to bind up in the threads after emtpying five or six chargers in a row — I’m not sure if that’s due to contraction or what, but brass doesn’t have that problem and its coefficient of thermal expansion is not much lower than Al’s. Its thermal conductivity is about half though… Maybe the sticky threads are due to condensation/frost and that doesn’t accumulate as fast on brass? Or wet brass is slipperier? I don’t know.
In any case, mating plastic and metal threads would definitely be problematic. And you’d still have to explain the special heavy-duty balloons you need with a cracker (which I have never seen used for anything else.)
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Flow control or no, inhaling directly out of the cartridge seems like a very bad idea.
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I agree 100%, a horrible idea, and should never be tried unless you like frost bite in your lungs. The flow control would be useful for filling up smaller balloons, plastic/paper bags, and the like, never for sucking on.
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