How is that worse than "they found pot under the floorboards"?
Worst case they get time off with pay while and internal investigation by their friends determine that procedures are followed.
You are trying to come up with technical solutions (oh install sewer traps). The problem is not technical. It legal, and regulatory. There is no, 0, nada, zilch repercussion for just throwing a flash grenade in barge in. Why install sewer traps and spends all that money? That is irrational from their point of view.
The floor boards and all that sewer stuff is just noise. All those "problems" are just a list hurdles they will never and or even think about solving, that was the main point.
However, I am just gathering facts to figure out what other options are available to police. If certain states, as a citizen I am judged very harshly if I don't exhaust all options before using lethal force, even if it's happening in the heat of the moment. I want the cops to do the same consideration of their other options, at least when it's an operation they get to plan out ahead of time to happen under the time of their choosing.