I have two "hoop" tractors made with chicken wire cedar 1"x3''s and PVC that fit on top of out raised beds. Ours are 4X6 and so light weight that I can easily move them by myself. They could be much longer and probably 5' wide and still be picked up and moved by two people. I found the instructions on the Internet. One tip though, the clips to attach the wire (or plastic for making cold frames) to the hoops is pricy but you can make your own by cutting a piece of pipe that is one size lager than the hoops into 1/2 inch slices and clipping about 1/4 inch away from the rings to form an opening that fits snuggly around the hoops. You could also use zip ties but the completed frame wouldn't be a rigid and harder to move. Ours has small door on one end.
The only down side to this system we have found is that it's detached from the coop and NOT predator proof so if you arrive home after dark the hens will be asleep and it's difficult to get them to go into the coop. It only took one time for us to remember to put them back into the yard before we leave if we think we might not be back home before dark..