Duct tape the guard to the box (this is a Peterson bluebird nest box). Since it's just a temporary thing, it will last long enough for our purposes, even if it is rained on. The bluebirds will be confused at first, but they will quickly figure out to fly around the guard to the hole. This is because they bond with the box after the first egg is laid, and they remember that they have a nest there. House wrens, on the other hand, have not bonded and so have no reason to try and get around the guard.