What could you use as a mini gunge tank? Maybe hold a cups worth of goo, to be used as a roulette type thing - hold above their heads and only release something like one in 6 attempts, or have multiple strings where only 1 is connected to the release?

Some flour sifters have bottoms in it that can be opened.. With just a few after market modifications, it could be turned into something like what you're describing.

Haha thanks Jay I wanted something that's gonna be the element of surprise... forfeit type thing - answer it wrong you face a possible gunking

you could buy a "Wet head" from amazon and replace the water with gunge, it a 1 in 6 chance of being gunged!

Had one of those before they're good but I want the gunge to be a bit thicker than it would allow