I read about plans for the same thing on an Roger Grahm's web site from Australia. All you need is a 2 lead type oxygen sensor and an Ohm/Voltmeter. Apparently you can buy used oxygen sensors fairly cheap from junk yards if you remove them yourself, but I priced new Bosch sensors with an extended probe at around 100.00USD.
There is a thread about this on clayart but the original (as far as I know) discussion and link to the PDF with the how to set up and "calibrate" is found here:
The Main page that is filled with Ooodles of good stuff is here;
I deleted a hobby and used the proceeds from the sale of the stuff related to said hobby to buy a fireball Oxyprobe, but was in the planning stages of using an oxygen sensor in the kiln exhaust when I remembered the RC airplane kits gathering dust in the attic...