Walmart sells a digital meat thermometer for pretty cheap. It's got a thermocouple in a stainless steel probe with a braided stainless jacket over the wires. I test it's relative accuracy in boiling water. Just snake the cord through the door, set the alarm to your bake temp and start the timer when the alarm goes off. Turn off the alarm and monitor the temp visually. I use one to set the digital controls of customer's ovens.