Ed Hayes wrote:
If have 2 feral cats in our yard which we feed and take care of.
Over the last year we actually can now touch and play with them but
they will not come in the house. They truly are wild.
This winter I would like to build them a place where they can stay
which will keep them relatively warm.
I can easily build them a small house and put a blanket in it but I
would like to insulate it and maybe add heat.
SNIP

Doubt you need a heater.
Had a very similar situation.
I "built" a house out of a sturdy cardboard box lying about
and glued 2" foamboard insulation to it.
Added an extra 2" on the bottom.
"Door" opening just big enough to allow cat to enter.
An old blanket on the floor inside for comfort.

Took a little while for him to trust going inside but
was warm as toast in minutes.

Jim