Such an obvious concept, I wonder why no one had thought of it before? A toddler with a kryptonite pebble could give Superman or Supergirl instant arthritis, so why not just build an entire container out of it? I suppose it would be like making a jail cell out of asbestos since the entire cage would be simultaneously killing any Kryptonian that was inside it. Thanks to a 1966 issue of Action Comics we get to see Brainiac using such an entrapment and we get to see Supergirl caught in a Kryptonite trap.
The story begins with Superman having to leave the planet and leaves Supergirl with the responsibility of protecting the entire planet by herself. She’s not even a member of the Justice League and yet she is left with this burden. The villain Brainiac shows up with a flying saucer, so naturally Supergirl needs to investigate. He is actually looking for Superman and threatens to blow up Metropolis if he doesn’t show up. Supergirl does what she and Superman always do, and that is try to solve the problem with their fists. When she tries to attack the flying saucer, it’s force-field knocks her all the way to the ground where she lands in an alley. She persists in her attempt to stop Brainiac as he tries all sorts of tactics, including a time-travel-ray in an attempt to knocker her into the far future.
Eventually Brainiac has no choice but to use a Kryptonite cage that he prepared for Superman. He first stuns her with a super powerful ray, and then snap-shuts the cage over her using remote-control magnets. The result is that we see Supergirl caught in a Kryptonite trap with now way out. To be extra-sure that she dies, Brainiac increases the radiation and releases Kryptonite gas. And finally he blasts the cage and its contents with unimaginable power, disintegrating everything. Despite being so thorough Supergirl still manages to escape by simply slipping through the bars. Since the cage was intended for Superman and not her, it was easy for her to leave. Brainiac escapes into another time-era.
If you would like to read this you are going to have to get Action Comics 339 and add it to your collection. This was back when comic books gave you real value by providing more than one story. The stories also would be told to completion in one issue, rather than spanning across multiple comics and multiple titles, forcing you to either pay a small fortune to collect them all or just wait for the graphic novel to come out. Tell me your thoughts in the comments.