You need a larger container to actually do the mixing. Some will use a pail, while others prefer to mix in the container used for storage.
I'm like you Thomas, I would choose to store the mixed solution in smaller containers, although there is a store nearby that sells 4 litre mylar bags for wine that look very interesting ....
I hear good things about the wine bladders.
To mix the chemistry I use a stainless steel gigantic cooking pot. I measure 4 liters of water at the correct temperature and pour it in.
Mix in the powders using a stainless steel stirrer.
Pour it back into a big graduate that can measure 5 liters, and top it up with water to the correct volume.
Distribute into smaller bottles.
I've always mixed my chemicals according to the pictures on the Kodak photo chemistry pouches, which show big open vessels into which you pour water and dry components to mix.
If I need to maintain higher temperature, like with mixing D76 or mixing/using C41, I just put the stainless pot on top of my heating plate, and it works for that purpose also. It's a good catch all solution for me.