And who's to say there's not some sort of catch under the edge of the lid ?
It's possible, but then how did Spock get out? It's not like there would be a catch inside, just in case somebody got locked in, it is a weapon after all.
It's probably safe to say that the inside was designed to be airtight, to ensure that none of the inner elements and chemicals would react before they were supposed to.
The way I see it, they removed all the inner parts of the tube and put Spock's body into it (whether this was within the reactor room due to radiation contaminant risks, or whether it was in sickbay is anybody's guess) and then sealed it shut, with the intention that it wasn't going to be opened with anything less than a cutting beam.