We made a large pvc cat catio using 10 ft sections of 3/4 " pvc pipe years ago and it still looks good as new.We attached it to a wood base to make it heavier lol
Due to the number of chickens you have and how big the tractor needs to be I suggest you make a pvc tractor 10 x 15 (pvc comes in 10' sticks).It would be easy to assemble and pull without wheels ( it would be light). They're popular with folks who raise meat birds on pasture because they can...