Like I wrote above, the mcmaster item number isPretty sure if going with carbon is to use a non metallic graphite carbon anode. McMaster-Carr has them.
https://www.mcmaster.com/7979A19/two 5/8 of an inch x 8" rods will work. Yeah price is a consideration that is why most folks use a junk piece of steel.
$28 plus shipping for (5) 3/4" diameter a 17" long rods.
They do not need to be similar length to the part being de-rustified.
I ran one rod for weeks convering rust on around 100 30+ round machine gun magazines from my brother-in-laws house of hoarding (with a leaky roof). I replaced the electroylte solution a few times (5 gallon plastic bucket filled practically to the rim). Rust was converted on the insides, outsides, and both ends of the magazines. It is not line of sight dependent. It is true that the anode is sacrificial and so the greater area of rust to convert, the faster the anode wears. When I was done with the first rod it was tapered down to 1/4" at one end and 3/8" at the end the lead was attached to. $7 shipped for that electrode.
Where there was flaky or furry rust I knocked the bulk off first with a stainless steel brush then hung the mags (Thompson and Grease Gun mags mostly) 2 at a time for 12 hours at 12V 6A in 5 gallons of water with 5 heaping tablespoons of Washing Soda. The parts could then be rinsed in water with a small swipe on the de-rusted surfaces with another stainless brush then left to dry. The did not re-rust where the conversion had taken place like parts processed with a carbon steel electrode do. For my purposes I hit them with some WD40 once they were dry (brother in law had 100 cans of WD spread around his house, in the hoarde, as well) and sold them with the SMGs