Mine is a 6x12 Haulmark. The silver cups are cat food bowls I got at the local dollar store.
They even have rubber on the bottom of them to help keep from sliding and are made of metal all for $1. The only problem is they are too deep for planes with wheelpants so I need to add something to the inside of them, I thought about pouring some epoxy in to do that.
The secod level has 2x4's mounted on each side into the frame, and then a brace across the front, middle and back and the sheets just sit on top with a couple screws to keep them from moving. I can remove them in about 10 minutes in case I want to use the trailer for say a motorcycle or a new big screen TV