I have been derusting a table saw cast iron table in a 40-gal Rubbermaid garbage can. I use an old steel shelf that had started to rust already as my anode, but have also used rotors. Any steel should be fine as long as it is not stainless as that may release hexavalent chromium which is the bad chemical that was in the movie Erin Brockovich. Line of sight is important, and many people will figure out ways to line their container with the sacrificial steel to improve coverage. Many modern battery chargers require a load which is why it is sometimes necessary to connect the setup to a car battery, but I find when the battery is not being charged, it will eventually discharge completely between the charging cycles so use a battery you don't particularly worry about. Depending on the amount of rust you're trying to remove, you may end up with a decent layer of black iron. Sometimes, it takes more elbow grease or a coarser abrasive to get back to bare metal, but with patience, you should be able to eliminate the rust.
You can get pretty creative with the container too. I've seen people use kiddie pools, 55-gal drums, I think someone even used a swimming pool that was no longer in use. The closer the sacrificial anode is to the item to be derusted the fast the process goes, but here, faster isn't always better. As long as you see some bubbles forming where there's rust, the process is moving forward. Washing soda is highly recommended. Baking soda is not the same and much slower. Graphite I hear is a great anode as it takes much longer to breakdown. With the steel anodes, you will need to scrape off the rust that gathers on it every so often. Here's a picture of my current setup and a smaller tank next to it that I used for smaller parts. Once you get setup, it's crazy fun.
Other observations, don't let your copper clamps touch the tank water as they will break down and eventually release not so healthy copper compounds into the water. If you use a wire to connect your sacrificial anodes, try to keep them out of the water as they too will break down (ask me how I know). I was using steel hangers to hold onto my old brake rotors but eventually I would have to fish them out by hand. I try to keep the rotor use to shallower tanks now unless I have a bendable steel rod handy.
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One method I observed but have not tried, someone soaked a towel in the washing soda solution and then connected an anode to it and laid it on top of a planer surface which had rusted pretty severely. He connected the black clamp (cathode) to the table itself and it effectively derusted anything that got wet from the towel. Pretty clever.
