I just got this small animal heating pad for my adopted outside cat Gritsy's outdoor kitty house. Grits won't sit still for a picture so here's a picture of a bunny from the Cozy Winters website instead.
The heating pad is made specifically for small animals and never gets over 102 degrees. It has a chew resistant cord and is actually activated by the weight of your pet. So, when Grits is running around chasing squirrels and getting pet by neighbors, the heating pad remains just a couple of degrees hotter than the air outside.
And it says right on the box that it's 'Great for cat houses'. Grits spent her first night with it last night and she seemed very happy this morning. Hope this helps you with your own outside cat.