what a plunger does is to get in the way of what your expensive instrument does.
Its still a expensive instrument, your simply using something to channel and modify its output. your hand on its own can work well if you just need a subtle effect.
personal technique is important when using a plunger of any type... unlike a stick in mute where you usually play as normal, using a plunder of any type requires a bit more player input.
sink plungers are more or less perfect ergonomically, sonically and finantially.
don't rule out a half coconut shell, I have one of those in my mute bag as well as a bowler hat.
Whatever works for the sound (and visual impact in more cases than not)
Sometimes we use a midget mute inside the bell whilst also using a plunger over the top, so a larger plunger is better in this event.
best alternative to a plunger (or any other) mute - no mute
Mutes should be discarded forever, they just distort the true sound.
Oh, and walking on stage to play Pagannini Variations in a contest as a trombone player with:
oh, and a trombone
is damn near impossible !