Here is what I do.
I do more than just tanks, so wanted to build a system that I could drop anything into.
I use a large plastic tote.
I ran lengths of rebar around the circumference of the tote. I drilled little holes on either side of the rebar, at the top of the tote, above the water line, then twist tied them to the tote.
I ran jumper wires from rebar to rebar to rebar, connected to POS on the charger.
Then connect the negative to whatever you drop into the tote, be it a tank, or any other rusty items. The benefit of this system, is that it gets at the rust on the inside and outside of the tank, and as stated, you can use it on other items other than tanks. The down side of this is it takes more soda of course, and more liquid, which is not a big deal, its cheap enough.