I just use a small plastic bucket with a float on it. The float is tied directly to my RO/DI. Both the bucket and the RO/DI are a shed outside. The water in the bucket is fed by a Tunze Osmolator through a hole in my wall (the tank is on the other side so the water is fed directly into the sump).
I don't worry about an accidental "on" since the Osmolator has an emergency override feature -- when the pump runs dry it shuts down the pump until it's reset. Even if the RO/DI is stuck on, it'll overflow the bucket in the shed, and I'll know that there is a problem since the Osmolator warns me.