I know that Bio will sometimes "babysit" and take them inside during the night and return them in the morning and the parents don't mind. Not sure how yours would react. I'd say if your judgement says its too cold to bring it inside.
Or, is it possible to put one of the parents on the nest (bring a flashlight so they can see) and get them to settle in for the night with the baby?
Well, they're already in my garage so it probably won't get too cold, but it could get as low as 50 in there. I'm about to head down there and check them again. Hopefully one of them is on the nest.