First time I saw a pic of the two-headed phaser, that is what clicked for me visually ... two garden-variety water hose metal nozzles, back to back, inside a pistol grip. It actually looks like existing plumbing pieces to me (though for all I know they look better from a distance in the movie, since I'm assuming they don't have a "Q gives briefing' closeup of the workings.)
The kill nozzle looks vaguely--vaguely--like one of those hose barbs, but you're correct that they don't get a closeup in the movie. And anyway, they don't look any more like a hose part than the nozzle on the TOS phaser looks like the eyepiece of a toy microscope, so I don't understand where this idea comes from that phasers should be above that sort of thing.