Rescue does not allow you to keep boxers outdoors and will not adopt to you if that is your purpose. Rescued boxers are meant to go to loving homes to be spoiled not banished to a backyard which by the way is how many spent their time before ending up in rescue.
Just get homeowner's insurance - that is what it's for.
If a dishonest person sees something they want in your yard - no dog is going to stop them. People entering a yard and letting your dog out is not the worst that can happen, even a police trained guard dog could be shot in a matter of seconds.