Stainless steel oven thermometer.
When the subject turns to analog oven thermometers and cooking in wood fired ovens; you will find out, if you haven’t already, that measuring temperature with these great self standing stainless steel thermometers is done/useful only when the wooden door is closed - that’s when temperature in the cooking chamber evens up for baking or long time slow roasting and so on. Otherwise there isn’t point to measure atmosphere inside while firing or when the entry door is opened because logically the temp is very different at vault level vs. bottom floor. Carbon burn-off gives you high pizza temperature reading, 
that’s when all the walls turn from black to white color (or to red color if the oven is build out of old red solids.) These meters sell for about 10 to 15 bucks in chef shops, I have about 150 of them left at the moment to give to my customers. I bought them for good bulk price from trade show in Brisbane.
If you prefer a fancier oven thermometer, or as another option although not at all necessarily needed, then take a look at digital thermometer.
June 13th
hello…. Im trying to build my first primitive clay oven and want to know if I can insert a brass well for a thermal probe or how to mesure temp without opening the door? any help? Thank you and god bless. robert.