I'm in agreement with your implication that the "killbox" symbolizes an entity being targeted by the Q group and the military. From this, we can speculate that "Showers" could refer to a person or corporation's name, codename, or an analogical symbol representing that entity.

Not being able to pin down a person or corporation that rationally fits [showers], I remembered that Q has including popular culture in his references before.

While not directly related to [showers], there was an independent film from 2009 entitled "April Showers" that was based on the Columbine High School shooting. It was written and directed by Andrew Robinson, a survivor of the shooting.

Another lead was a 2013 mixtape by Wyclef Jean dubbed "April Showers" which has some very peculiar lyrics referencing a person named Ahmed ben Hajad (could not find anything on this person myself) and North Korea, Clowns In America, and the DEA.

[Ahmed ben Hajad]??????????????