Add a top center hasp. Hasps are hardened steel and installed with carriage bolts through the door or sate. Ask your hardware dealer about them. There are many types of alarms on the market. Many are very expensive, though some fairly inexpensive models are now sold.
Most alarm systems have their limitations and are god only in certain applications.
Don't depend upon an alarm to protect you completely. Any alarm should be used only in conjunction with other good, common sense crime prevention practices and with the proper locks and lighting. Before buying an alarm system, make sure the company is licensed to do business in North Carolina. It's also a good idea to get cost estimates and references from several reputable firms. Check with the Better Business Bureau as an added precaution.