I'm sorry, but I just checked and there's no thread posted in the X - 5 section of the www.4wdrc.com
forum regarding the battery strap.
Most guys who use a different strap for the left side use a Shumacher strap, but I don't have the part number.
We tried two different types of rubber, both black, for the left side and neither of them worked. One was too stretchy and the batteries fell out, the second broke where the nut & washer hold it in the car.
The brown one we supply is ugly, but it works really well. Holds the batteries tight and actually is not affected much by brief contact with a soldering iron or hot solder, something that happens from time to time when installing batteries.
For a Shooey part number, I suggest you post on the 4wdrc forum in the section for one of our English drivers.