build your own Inboard Engine Motor Flusher Fake a Lake Inboard Motor Flusher

You can build your own using a $1 toilet plunger, a garden hose adapter and a piece of garden hose.
Takes about 2 minutes of work and works as well as the store one.
The only disadvantage is that is't not height adjustable so you will need to support it so it clamp on the boat.