I bought two one gallon and one 3 gallon plastic water jugs from Walmart.
They are clear tinted blue with a white cap on the top and a spout on the side with the handle.
I used a stepper drill bit to get the right size hole for RODI fittings from local hardware store and some rodi hard tube for the pick up tubes.
Worked great and held plenty of fluid for what ever I needed to store and dose.
Wish I still had a picture, you can find them in the tupper ware section near the ice tea pitchers and such, usually on the bottom shelf and they cost just a few bucks each.
The one gallons are slim too, the only thing is You need the space for them to sit on their sides due to the design. I found the side spout/ spigot was useful for manually adjusting levels in the tank or another tank.