I just looked at lots of photos on here and the Internet and designed what I wanted. Different things work for different people and locations.
Look through some posts or do a search for tractors. You will find a lot on here.
Check out my BYC page and the M*A*S*H page I have.
I don't have plans, exactly, but I will let you know that after three months of agonizing, doodling, and going broke on graph paper*, I choose to do my tractor-for-seven as a two-parter (coop on 4 wheels; lightweight attachable run). Just about anything else either weighed too much, or didn't have enough room for them all
*Seriously, some of the sketches I did would put Da Vinci to shame