I have 2 bricks inside the tub, the waterer sits on top of the bricks and the tub fills up to the hole and stops. The bottom stays off. The bricks are the base.
So I was browsing on Pinterest and I came across this post. I bought this water at tractor supply and I have a rubber tub like this already. Yay!
Link below...
http://frankiemakes.blogspot.com/2012/06/watering-solution-for-ducks.html?m=1