First, I would suggest you get you a battery pack, this will alleviate the cells in the battery box from slipping off the contact points and causing a runaway. It will happen and from your post it sounds like one might not be far off for you.
Once that happens your engine is toast, it will still run but the life of the engine is gone.


To answer the question, the best way is to use zip ties. Thats the way the battery holder in the back of the chassis was designed to work.
You could try rubber bands but I doubt that would hold very well.

Get a pack and a nice switch that has a charge jack on it and you wont have to take the pack out to charge it.


Dbow