By the way, while I have not (as of yet) had to do this, as I have always been able to get ahold of 8 pounders to do my work, I have considered this as a possible need to mix powders from different 1 pound cans in the future - as you are doing now.
So, I wonder if there are other suitable and readily available jugs on the market now for such applications. I have thought about buying store-bought gallon sized distilled water (hopefully little or no contaminants present) then thoroughly drying the empty jug for use such as this.
Just a thought.