Great job! You may consider buying "rescue blankets" on amazon.c

The rescue blanket is made of foil.  Usually people use it for emergency situations, such as when you are stuck in cold weather.  So it's big enough to cover a human.  You may cover the inside of their little houses with the rescue blanket.  It reflects majority of the body heat, which is crucial for retaining heat.  But you need to make sure the kitties don't chew the foil.