RS4, here's a quick way to hold the batteries tightly:

Go to Walmart and look around in the craft section for "industrial strength" self adhesive velcro. It comes in rolls 2" wide by 3' long, IIRC. Put a strip of the hook side on your chassis, and a matching strip on your battery. The industrial strength velcro adhesive backing will NOT let go from the chassis or battery. And as for the velcro itself, it will be strong enough that you will have to twist the battery against the chassis or slide a credit card between the hook and loop sides to release it. You can also use the straps for added security.

I trust this method to hold the batteries securely in my aerobatic electric planes, so I'm sure it will be plenty secure in a Stampede.