I use some 5 gallon containers I got for $6 at my LFS that I keep my RO in, I try to keep 2 5 gallon containers of RO and 1 5 gallon of RO I use each week for my automatic top off.
I use a large rubermaid trash container that is new...to mix salt water. I start the mix on Thursday evening and do my water change on Sunday. I know that many don't do that, but now it is habit.....and habits are hard to break. I do believe that you need to mix the saltwater 24 hours before you use it. I use 2 powerheads to keep the water moving and mixing. You only need one.