I finished the PID temperature controller for my Lee Pro 4-20 pot a while back. I don't know how I was ever able to cast anything without it.
You can see that it makes a liar out of that stem thermometer.
The controller and SSR cost $32.99 and the thermocouple cost $6.39, both from evilbay.
The single ac outlet was about $6 from Lowe's.
The fuseholder and switch cost about $10 from a mail order surplus dealer.
The cabinet is an old printer a/b switch from the junkbox.
The rest of it is junk box salvage stuff, but I'm betting it wouldn't have cost more than $20-$30 if you shopped around.
I could have added additional features like a PID bypass switch for heatup, and a maybe a few indicator lights, but I wanted to keep it down and dirty, and simple.
A heat sink has not been necessary. I mounted the SSR directly to the bottom of the cabinet with thermal compound. The cabinet only gets slightly warm. If this controller was driving a higher current pot, heat sinking might be needed.
I did not bypass the thermostat in the pot. It serves as some protection against temperature runaway, if something goes wrong with the controller.
The most difficult and time consuming part of the project was fabricating all the hardware items.
Wiring was dirt simple.
The only issue is that you have to use crimp type connectors on the chromel/alumel wires from the thermocouple. They won't take solder. So hooking them up to the connector took a little extra doing.
No modifications were required to the lead pot except for the mount bracket for the Thermocouple. Just plug the pot into the socket on the PID and turn the thermostat on the pot on high.