Existing Velcro strap works perfectly with EX and kit graphite battery holders with no modification to the strap...
All you need is some large spacers, a pack of the TOP suspension mount spacers work perfectly.
Thread Velcro through one end all of the way to the loop so that the loop is on the battery side. Secure using the 1mm spacers and mount to the chassis.
Thread the Velcro through the other end but build using some of the large suspension mount spacers. This will give some room so that the Velcro will move when you are inserting / removing the battery.
When you insert the battery you will find that it sits right up against the loop further securing it meaning that the Velcro will never come off.