Install a bar on a hinge, midway up the door. you can buy this at most home improvement stores.
You may have seen this on hotel or motel doors.
The best part about this type of lock is that it can be seen by an intruder before they try to brake in and they may move on to an easier target.
Try http://www.crlaurence.com if you can’t find it at a local store.
Ps. If you are using a bar on the bottom track, you should drill a hole and insert a nail or pin that you can remove to keep someone from banging the bar out of the track.