I use a 55 gallon plastic drum, float valve installed at the top to auto shut off the RO/DI unit. On the bottom i have 2 bulkheads, one goes to a MAG7 to fill up a 35gallon drum i make saltwater in, the other goes through a john guest valve then to a float valve in my sump that opens up when evaporation hits during the day... it makes top offs a thing of the past....
Jasen