Can anyone point me in the direction of a cheap auto shut off water level valve?

I am not looking for auto top off for my tank right now. But I have a decent RODI system setup in the basement. I have a 50G white tank, and a Rubbermaid 20gallon. I use one for storing RODI and one for making up salt water. Anyhow, I'm just looking for peace of mind when filling the RODI. As of now I turn it on and try to tell my phone to remind me to shut it off. If I forget or miss the reminder I'm in trouble.



So I'm looking for a really cheap solution that doesn't have to be fool proof but just a backup. Any suggestions?