Try this method (discovered by my friend):
Take two pieces of 1/2" or so PVC pipe and cut them to a length so they are a little longer than the glass break's mounting distance from the floor.
Place the two pieces so they are vertical in front of the glass break about six inches or less away from it. Separate the top ends while keeping the bottom ends together. Now slap the pieces together.
If you have carpet, set the pipes on a piece of wood (so the sound doesn't get muffled).
You will be surprised how well this works.