Get a big plastic tank filled with water. Take a wide board large enough to fit across the tank and cut out a rectangular piece from the middle for the seat. Attach the seat to the support board with heacy duty hinges at the back so it can swing down. Attach a heavy duty door latch of the type the public bathrooms use underside the seat so that when the latch is closed the seat cannot swing down. Horizontally mount a bicycle wheel on the far side of the tank. It should be mounted on the same side that the latch is going to be on. Mount it so it can spin freely. Create a target. You can use a piece of wood, a caricature, or anything else. This is something for people to throw at. Nail the target at the end of the lever arm. Attach a large lever arm to the bicycle wheel. This is the board that the target is going to be mounted on, so it can reach out far to the side of the wheel. Cover the seat with something comfortable. Attach the longer board across the top of the tank so that the other board swings downward when released. Attach a string from the bicycle wheel to the latch so that, when the lever arm swings back the latch pulls out and releases the seat. Fill it with water and try it out.
You can have fun and be creative with your dunk tank. If you want to know how to build a dunk tank, you can get the iron, wood, and plastic. You can rental a portable one for less. You can even fill it with plastic balls, instead of water.