We started designing this for timber. Some second hand Australian hardwood. When we priced this though it was pretty expensive. We investigated new steel and decided to use that instead. One of our friends kindly lent us his mig welder.
The photo above shows the raw materials. We have 2 sheets of 25mm galvanised steel mesh. 2 sheets of old corrugated iron. Our local steel market cut the 25mm structural steel to length for us. We based the size of the cage on the size of the mesh, 2m * 1.2m. The structural steel is then 4 * 2m lengths, 6 * 1.15m and 6 * 0.55m (height).

This photo shows Dad doing some welding.
This is our basic box structure. We laid out one rectangle, measured the diagonals to get it square, tacked it in place and then welded all the joints. Mig welding is so much better than stick welding, and the light sensitive mask we borrowed was amazing. With a bit more use of it we would get better. We would also measure more often to ensure a plumb structure.

Here we have welded on the mesh. The bottom of the cage is an uncut sheet of mesh. The sides are 1m * 0.6m, the end is 1.2 * 0.6m. We determined the height based on cutting a sheet of mesh in half. This also gives ample headroom for the rabbits.

Next the corrugated iron. We pop riveted the iron onto the frame. One section is hinged to get access to the rabbits. We also cut some rubber tubing and slid it over the corrugated iron edges. We pop riveted these on too. These are just to protect us and kids from the sharp edges.
Before attaching the iron we also painted the whole frame with kill rust, just to give the cage a bit more life.
Pop rivets are amazingly useful, I had bought screws for the hinges but a couple of pop rivets held them fast.

Here it is in use. The wood holds down the hinged roof. The PVC pipe is for the smaller bunnies to hide. To move the cages I lift one end and slide a round bit of timber under the cage. I then pull the cage over this wood to the next section of grass. The cage to the left is our first attempt and has mesh on half the roof instead of a complete roof of iron. We get a bit of rain here which necessitated the extra cover. The breeze circulates well in both models. I put extra grass in with them, and other treats like carrots and corn, harder food like privet branches are good for keeping their teeth down.
They get their water through a drip feeder, they have rabbit pellets in a custom made PVC pipe attached to the side of the cage.
I will do a report soon on the growing process.