DDW_OR, it's nothing fancy by any means. I cut down wire cattle panels to fit and bent the wire "spur" around the next panel at the corners for support. Gate is connected the same way with one side closed with wire or spring latches. The board that slides in the back is critical to keep critters from going under the gate.
This shows it with the top off for collecting bagged leaves for garden mulch. Pulled by the Kia Rio I used to have. Pulled my first 7 gilts home with this car. 3.5 hours one way.
Usually it's pulled by the minivan now. Wire and ratchet straps to hold the rack down. Easily removable.
This trailer and hitches on the minivan/Rio have saved me tons over the years over buying a truck+insurance etc. I do have a truck now (F350), but they still save a lot of money in gas over taking the truck for small loads. Plus I can take ALL the kids and don't have to move carseats around.