Battletrap is one of mine too. My preference re the bacpack is to use the big chunky tab/slot that's used in copter mode to lock the arms together to secure them as a backpack. You end up with the chopper blade between the torso and backpack, but it's doesn't flop about.
I also prefer to bend Roadtrap's arms 90 degrees at the elbow in combined mode. Otherwise I find that the arms interfere with the ankle articulation.