Boxer is a good option, but i did want to throw out that I do think goldens are more protective that you might think. While friendly dogs, they will bark at strangers around your house, same with labs, so don't rule them out because of that. The labs and goldens that i know are way more protective than my husky/weim mix (although neither of those breeds are particularly protective). I never thought i liked goldens too much, but once i started dog sitting for a couple, i fell in love. Plenty of golden and lab mixes at shelters too!