First of all, welcome to the Gauge. You'll get your answer, you can bet on it, but right now since I haven't got any of these, I've no clue. A picture would be helpful, and if you don't know how to post pictures here, go to the Photo forum section here and there is a step-by-step thread.
Could these parts be something that secures the containers to the car, or to each other? Just a guess since I have no idea how big these parts are, but hang tight, someone will be by that knows.