If you have any concerns about other animals using the door (from feral cats to raccoons), there's a clever innovation available. Some doors have locks, and the key (magnetic, I think) is on the animal's collar. You'd need to use one of the breakaway type collars for their safety (in case the collar gets hung up on something). If they are staying outside or in the garage, you might need a way to keep other animals out of their food.
You might want to add a "nest" or two that they can use for additional warmth inside the garage... there are a lot of designs out there, but a cardboard box or two with an old blanket or something so they're not sleeping right on the concrete floor would probably do the trick.
