I was thinking of this. I have a dog that fits in a kids trailer after I took out the seat fabric and put a big pillow in it. Sometimes her brothers come to visit and I have up to 4 dogs. Luckily, I have 8 feet of trailer with my bikes to work. My dog did take some time to get used to the trailer and she would have tried to jump off if she could. I was thinking of plastic plumbing pipe for the frame and just wire mesh to keep them safely enclosed. By the way, she now is very happy waiting in her trailer when I go to a restaurant or something. She hates to be left at home (she rarely is).