If you preheat the mash tun with hot water first, and you insulate the mash tun, you shouldn't lose more than a degree or two during the one hour mash.
I use 175ish degree water in my MLT, and cover it. After it's heated, I take off the cover and when it lowers to my strike temperature, I add the grain. That seems to really "hold" the temperatures well, in my cooler MLT.