Probably the best purchase we made for winter time peace of mind is this luxurious Kitty Camper (we bought it at our local pet food store, Mud Bay, but if you want to google it use this: "K&H Pet Products Outdoor Heated Kitty Camper 14x20in").
We started with the heated mat, which you can actually see underneath the camper in the picture (ps: I didn't put it like that, someone was redecorating). But, when it started to get really cold we started to feel bad. Were the warm enough? I'm sure they were, but, we got the Kitty Camper anyway and haven't regretted it since.
There is a shelf hanging off the barn wall where all of this sits, and we wired to put an outlet right under it so there's no excess cord material. Extension cords are one of those things that drive.me.nuts.
And so Boots and Izzy are nice and toasty all night long. And, as a bonus, if they're outside during the day and get a little rain soaked they can come in and warm up.
Of course, when they ask to come inside with their people we don't say no...there's something really wonderful about having all six cats in one room, but, as soon as they ask to go back to the barn we quickly oblige...because if we don't, they start destroying our furniture :) And they are, after all, barn cats <3