Why the need for insulation ? If you're doing BIAB (as I suspect - the bag;s in a photo) couldn't you just place a wire trivet into the keg so the bag isn't touching the base, then spark up the burner a couple of times through the mash to keep the temperature up ?
I use a pot that only just fits into the oven, so I can keep it warm that way, but am soon stepping up to a larger capacity, and didn't plan on insulation, but rather the method described above.