At intersections, drill the pipes with holes just wide enough to thread strong wire through, then twist the wire firmly to create a strong joint.
A few cross braces (from bottom corner to middle top) keep the thing straight.
Add chooks, a moveable roost, water and feeder and hey presto...
The hardest thing to get right when building tractors is the balance between sturdiness, floor area and heaviness. Too light, and the unit is flimsy to predators. Too heavy and it's a drag to move. Too small, and it's either got to be moved daily, or the birds feel overcrowded and unhappy.
The above unit is strong enough to be safe from foxes (see the point below about a mesh skirt) and yet light enough for me to move weekly (which given the large size — 3.6 x 2.7 — gives up to 10 birds a good amount of green forage). The high shade trees in the tractored area are a necessity as chooks can easily die in tractors in full summer sun; but I've also put shade cloth underneath the tarp, and when it's too hot I simply go outside with some loppers and cut tree branches to lay over the top. It works, and performs well as a 'grower' pen for birds as young as 6 weeks.
An important note about fox proofing: after the above photos were taken, I added a moveable skirt (separate from the tractor) of 30cm wide welded mesh strips, laid flat on the ground around the perimeter of the tractor. Both the perimeter mesh strips and the tractor itself are firmly tent-pegged to the ground. This tends to make the unit quite safe from anything that wants to dig to get in.
Incidentally, we did have one enormous blow that began to lift the tarped end of the tractor off its moorings... But that was an extremely unusual weather event that also brought down some big trees in my block and next door. The chooks were so used to the tractor they simply stayed put and waited for the rear end to come back to ground. I've since found slightly longer tent pegs and don't expect the unit to lift again.
