At My Tenth Life we primarily use straw for warmth. I don't know if you've ever slept in a pile of straw but I can report that it is very, very warm. There are more straw houses here than are needed to accomodate any combination of feline friendships that may be taking place on any given day.
There is also the 'Senior Compound' which has recently been vacated by Checkerboard and Grace moving inside. I have kept the heater on and put fresh pillow slips filled with straw inside to attract its next inhabitants.
The heated house is close to the building. Most of my ferals have their snuggle spots set up perfectly for themselves and are reluctant to move. Still, I'll keep the heat on and the straw fresh.
I welcome input as to other house types that you and your cats find valuable.