Backups are good...but consider this...
The glass surrounding heaters is CONSIDERABLY thinner than the glass of your tank...what would happen to your heater if a rock were to topple on it...well, naturally it would break...now, you have an exposed electrical element in your tank water and your heater decides to cut on...not a pretty thought...
Long story short, just put the heater in one of the rear chambers...
Edit - Remember, it has already happend NUMEROUS times before to people and there are several threads showing the bad results.