I use the duratrax temp gauge and I compared it to a $350 infrared temp gauge I use at work. It was only 5 degrees off. The gauge is used as a reference and should not be a used as a critical tool when tuning the engine. Just make sure there is always some smoke/oil coming out the exhaust pipe and you keep it tuned to operate at peak efficiency, that way you don't have to run WOT as much and don't overheat the engine. Is the carb set at the stock break in settings? These are the only settings I would use for the first couple of tanks at break in. The manufacturers test their engines and determine the best settings for break in procedures, so I would trust what they say to use. If stock settings don't allow the engine to get up to temp, then use the foil or sock method to get it up to temp. Or if it's due to wind blowing across the fins, just block the wind and it should get up to temp.

I would use aluminum foil, that way you don't risk having a sock on fire over your engine and you can always poke holes in the foil to allow a little airflow if the engine gets too hot. Of you could just throw a burger on it, then you can break in the engine and cook lunch at the same time.