I wouldn't be leaving her outside. Boxers aren't equipped for the cold at all, plus there are a lot of other risks involved, including theft. She'd probably be happier in a crate. Part of owning a dog is sacrificing part of your house to an ugly black eyesore. Thank goodness my dane is trustworthy alone inside, because those crates are terrible. BUT they serve a very important purpose.
Anyway, Muttluks and Chilly Dogs (Chilly Dogs - Outdoor gear for Active Dogs - Dog Coats
) are great for the winter. Just please keep her inside when you're not home.