For our boys we have snow rated Exped self inflating mattresses and then we purchased a rentangle wool blanket thing (which we got from an ugg boot shop), which we then lay on top of the mattress and under their protector and base sheet, then on top they have a sheet, a wool blanket, and a down sleeping bag opened out. Not once had they complained of being cold.
We've thought about a portable heater of some sort but don't want to risk it.