I would not suggest an IR gun in this case it is a fantastic tool (I own 4 with different DS) but as with all tools you need to understand the limitations a translucent plastic is not likely to produce accurate readings (emissivity is way off of the calibration point) and it is reflective so it can reflect the reading. I would place a probe at the back wait 30 min and take the reading and double check the ambient air temp wile you are at it. (although if memory serves you are in a warm country so this isn't a problem. ? or do I not recall correctly?)

Small demonstration, an accurate IR gun measuring a target very different emissivity. It is reading 12ºF off. (23.4 is the accurate reading there is a highly accurate traceable thermometer under my hand (the large box one).

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