If you choose to alter your air popper you are assume the risk that it will no longer work as it was designed to, that it might not work at all, that you might injure yourself in performing these tasks or in using the altered device. This information is here for you to peruse, but Sweet Maria's accepts NO responsibility in what the results may be.
A thermometer can help you establish a more consistent roasting routine. Adding a thermometer to your air popper measures the temperature of the forced air heat flow after it has passed through the beans. Therefore, it is neither an empirically accurate measurement of the bean temperature, nor of the roasting chamber. That means that a guy roasting in Chicago with a UEI brand thermometer in his Popcorn Pumper and someone from Tulsa with a Pelouze thermometer in a Poppery II cannot accurately compare their roasting notes. It does mean that the first guy can observe that his Sumatra reached a higher final temperature in 5 minutes than his Harar, or that 2 ounces roasts hotter than 3 ounces, etc.
We sell a Update pocket type thermometer which is a good value. It has a 5" shaft which makes it ideal for the Whirley-Pop and adequate for the air poppers. As the beans lose their moisture and begin to rise, they meet the 5" shaft and your readings will be accurate. It's always in the airstream, anyway. The 400 degree candy/deep fry thermometers are widely available too but I don't like extrapollating the measurements between 400-500 degrees, since that's where temperature is critical. 500 degree thermometers with longer shafts are available, and would be ideal for the air popper but they are prohibitively expensive.
We found that this installation method, with or without the thermometer clip, made the thermometer wobble around a lot in the air stream and as the beans came into contact with the shaft. One possible variation is to simply fix the thermometer to the hood with the RTV type sealant mentioned below. But we like to remove the thermometer to use in our other roasters, or for cleaning.
So we searched far and wide for some sort of metal fitting, ideally a sort of collared bearing with a locknut to keep the thermometer in place, but allow for its removal. We couldn't find anything that would fit the bill. So we compropmised by modifying a common fastener called a Tee Nut, and adhering it with high temperature RTV sealant.