I purchased the kit from EI to do the 4-ch upgrade. I had been trying to cobble it together myself thinking I could use a locally sourced switch, but I was told that the rotary switch is a special one for thermocouple wire, so I abandoned my solo effort and got the kit. I have yet to install it, but it is pretty nice with all the connectors made up for you.
I do have 3 or 4 CHT probes that I can part with if you need them.
Let me know