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This might sound like a strange question, but has anyone ever heard of a plunger type mute for tuba ? What would you use if this was written for by a composer ? Would a harmon produce the same kind of effect ?

I've always wanted one but I suppose since no toilet that large exists the plunger manufacturers don't find it very important to make an 18" plunger cup

Not quite the same thing, but ...
If you watch videos of the Mnozil brass, you might see that the tuba player has a cage thing on his bell with a string that goes up over the top. The string is attached to a mute, and he can raise and lower it by pulling the string.

I used to use a plastic washtub.
Now I use a plexiglass thing about the size of a frying pan, made to be placed over a plate of food in a restaurant to keep it warm. The edge has to be covered with something soft to keep it from scratching up the tuba bell. (I use denim from an old pair of jeans.)

I made one out of one of those large, plastic, fake "terra cotta" dishes, that a flower pot sits in.

Put a handle on it, and used the soft half of a velcro strip, where it contacts the bell.

The added advantage is that it is a similar color to a real plunger.....

I'd use a crash cymbal.