I can think of three options.
Can you put larger items in the back like the milk carton that can't fall through? That would block and keep everything else from falling through as well.
Not free but try these to make a back wall on the shelves. Clamp them upside down on the back of the shelf and screw a piece of cheap 1/2 pine or even a cut off piece of clear plastic on the back. The idea is to put a back lip that runs across the shelf so even if it slides forward things can't slide off the back.
AmazonA bit uglier, buy a sheet of insulating foam at Home Depot and cut to stand on end and fill the gap between the door and the shelf. Then put the foam sheet into the refrigerator and close the door when traveling.
