I use 4" PVC couplers with 16oz deli cup inserts for water. It's handy because you can just replace the deli cup instead of the whole thing. Some people will hot glue these to the insides of the tubs. I don't do this because it would limit the type of substrate you can use (I prefer newspaper) And it's really not necessary. The snakes will move the water bowls around, but I can't actually remember ever having one knocked over.

Mark