I took a 15 gallon keg, cut out the top (like for a boil kettle), drilled a hole in the bottom. I welded a 1/2 inch threaded coupling over this hole, and installed a valve. Inside, I put a circular false bottom. I had to cut slots in my top opening to accommodate the screen's installation and removal.
I built a wooden box, roughly a cube, around the keg and filled it with Great Stuff spray in insulation. When I use it, my temps only drop about one degree in an hour.
My one mistake in engineering the Mashtun was welding another coupling in the side to accept a dial thermometer. I made the hole too high, so when I'm doing a five gallon batch, the mash is actually below my thermo.