An alternative method would be to have the backpack or whatever accessory set up as it's own .glm file and then tag it to the clonetrooper's body. I believe 'chestg' is the tag surface that jetpacks are bolted to, not sure. This is how the game attaches jetpacks, sabers, guns, etc to your character whenever you're in game - by bolting a separate item to a part of the character model, such as the back or the right hand, etc.