If you are doing a single temperature infusion, once you get the mash stirred and have the temperature where you want it, put the lid (preferably and insulated lid) on the kettle and leave it in placed. Even a DIY insulated kettle isn't going to hold temperatures as well as a cooler unless you are adding heat to your mash liquor somehow. I insulated my mash kettle with closed cell foam wrapped in aluminum foil tape (including the lid and the bottom. I would lose several degrees in the first 30 minutes according to my kettle thermometer (never opened the lid). Once I began to recirculate, the temperatures really dropped off. After several batches, I revamped the system so I could heat the mashtun. The only insulation I have now is on the kettle lid (because I was too lazy to remove it). Temps are stable within 0.5 degrees.