Hey guys, it's no harder than to put another servo mount with screw in the rearward center holes as well if you want to have a support there. This way, using the front screw in the servo mount and this rearward screw, you can even adjust the battery position sideways. Or just put a some weight there with doublesided tape, as someone already suggested. Those are 2 solutions if you do not want to buy any special part from the old car.
The battery support with servo mounts works very well, I have been using it for 2 years without one single problem. It is a very neat and simple solution that lets you adjust according to battery size (remember to leave a small gap to not tweak the car) and battery position.
Don't make a problem out of a non-problem. At least not until you have tested it.