One way to tell if the battery movement will be a problem is to try balancing the airplane at the specified CG with the battery at both extremes of its front-to-back range of motion. If, with the battery as far back as it can move, the plane balances out tail heavy, then it will definitely be a problem.

You should probably secure the pack regardless. Stuff a little foam rubber in the battery bay to cushion and hold the pack in place. It won't melt, unless your batteries are coming out REALLY hot, too hot to hold. Batteries that hot indicate a problem.

If your batteries are too hot to hold coming out of the plane, you're drawing more current from them than they can handle. The only time this is okay is if you're doing it on purpose, hot-rodding your Projeti or competing in F5B, and you don't care if you fry the pack. Otherwise, you either need to reduce current draw on the pack by reducing the propeller load on the motor, or you need to get a different pack capable of producing more current.