I did this to both my HLT and MLT, and they work great. FWIW, Harbor Freight has this type of thermometer pretty cheap, and I've found them to be more durable than the tru-temp/taylor that cost twice as much.
I've got a 1/4" SS sight glass+thermometer T-fitting from Northern Brewer that has never been used, with a thermometer port, if you're interested. $25.00 and it fits a 10G cooler.