5 wraps of reflectix made into a jacket, seals up with some thick velcro straps.
Bottom cavity of keggle filled with spray-foam. (Tinfoil separating the foam from the keg, so I can tear it out if I ever want to without foam sticking to the keg)
2 lengths of black foam pipe-wrap around the opening. 2 circles of this above the keggle-cut, and one around the top rim to make a place to slide in a plastic disk into the gap. Seals super tight.
3-4 towels over the top, bunched up on the top to give 5-6 inches of insulation over the plastic disk (disk is actually the bottom of a wal-mart cake-carrier)
Wrap the whole mess up with a blanket if there is wind.
I do this in my gargage, and lose MAYBE a degree if it's very cold.
I do only 10-13 gallon batches.
I love my keggle mash tun. Make sure you get a good false bottom and a good paddle.