Okay, these are just photos of LOTS of tractors from a yahoo search for "chicken tractor designs".
http://images.search.yahoo.com/search/images?_adv_prop=image&fr=ipad&va=chicken+tractor+designs
As you can see, the designs are endless in variations! But as you look them over think of several things that affect the design decisions:
Ventilation: very very important! nighttime poops are large and stinky, which is bad for chicken lungs
Weather protection: would you want to hang out in there in the heat, cold, or rain?
Predator protection: what do you have in your area? Dogs? Cats? Eagles? Diggers? Animals with small arms that can rip a chicken head off? Snakes? Rodents?
Weight: better protection means heavier tractor: will you be moving it by hand by yourself? Using a powered vehicle to drag it?
Wheels: they need to be able to fold up so the tractor will sit on the ground
Location: how flat is your land? A problem is land that is not nice and level. The gaps can let predators in and chickens out. If the bottom is fenced, the birds cannot walk, or reach the plants and bugs.
Where will you put their water supply so it is easy for you to fill and clean, and so they cannot spill it? I have one chicken who will purposely tip over any type of container. I had to put a hanging nipple waterer, and I also put a cup in the hollow of a concrete block.
How will you access the inside for cleaning and repairs?
Where will the roost go?
Where will the nest box go?
Finally, imagine you have an emergency and you cannot be home to lock things up... How likely is it you will come home to find a box full of chicken parts?