Plastic or Laserlite is a doddle to work with but doesn't last like ZAl, or indeed as Dave pointed out, protect nearly so well.
Bunnings sells stuff called "ripple" sheeting which is quite lite weight and easier to work with than full 3mm roofing. The downside is, its pretty dear (about $17.00 for a 1mtre x .5 metre sheet - more for bigger).
I'm going all tradish and making mine out of white weatherboard & this stuff.
All my other designs look like dunnies.
