This 6x16 tractor is easy-to-build and very sturdy. It's built on 2x6 treated wood runners. The canvas sides can be partially rolled-up in Summer to relieve heat. It will comfortably house 25 birds. 10 nests are two-tiered (5 on top and 5 below), are at the rear and accessed through the hinged doors, as shown in the picture.
(Boards around the tractor were put there when it was first built to make sure dogs would not try to tunnel in.)