When i do partial mashes I put a cake rack holder thingy in the bottom, grain bag sits on it about 1.5 inches off the bottom and put the pot on a small burner which is on low. My temp stays at 150 the entire time. I open the lid and stir mash about every 10 mins as well as rotate the pot a quarter turn every 5 mins or so.
I was losing heat quickly any other way I tried it, wrapped with blankets, you name it. Never put it in the oven but this current method works great.