I have a 5g jug sitting over my sump. It's plumbed to the RODI in the basement, and gravity fed to a float valve in the return of my sump (lower line in pic).
I have to manually refill the jug every 10 days by turning the valves and water supply on at the basement.
Not the most hands off approach, but at worst if the float valve fails, I only lose 5g into the system (sump has 6g free space). Most often I forget to turn off to RODI and it overfills the jug, and spills into the sump.which is why there is also a water alarm to sound when the sump is 2g from overflow.
I call this the Sasquatch method.
Since his bucket idea led me to this , even though his has an extra failsafe overflow catch bucket.