Hi:
The connecting part for the two tray halves is made of metal, black rubber and hard white plastic, and is 7/8th inches in length. At the top of it is a slot for a screwdriver to turn it to expand the rubber section and hold it in place. I should have enough time on the weekend to take a picture and post it.
That being said, I don't have it hooked up in mine. As long as I don't flip the car over, gravity and friction hold the upper tray in place quite nicely by themselves.