I use a digital meat thermometer with a probe. If you use this kind make sure you only submerge the probe and not the cable into the mash, or it will give you the wrong readings. In this respect, fixing it to the side of the mash tun is fine. Also make sure you stir it well and the mash is thick so that the temperature is the same throughout the mash. With a thin mash you get 'hotspots', I find.