Hi! First post.
What I did was take a milk crate, and using a table saw, cut the sides down to about 3". Then I cut one of the sides off, leaving me with a little pickup bed, open at the back. I ziptied it to the top of the rack, and it's rock solid. With a couple bungee cords, it's way more useful than a skinny rack, or a full size milk carton.