Here's the key: a thermoMETER measures the temperature; a thermoSTAT controls the temperature. Similar words with different purposes.
Example: Say you set your thermostat to 90, but you use the gun and the spot right above your UTH only reads 87. In that case, turn your thermostat up 3 to get your desired temperature of 90 above your hot spot. Think of a thermostat as more of a set point rather than an actual temperature.
The one I linked to is handheld and you use it to measure temperatures of whatever you point it at whenever you choose to do so. Ideally, you want to monitor every surface your BP could come into contact with inside its enclosure. A temp gun allows you to do so. If you do decide to go with an inexpensive thermostat, it becomes even more important to use that temp gun to measure temperatures more frequently to make sure your stat is functioning correctly/safely (within the range you have set it to be).