One more modification to the warm/cold brooder below: covering it with mouse mesh.
I came home from shops to find all the chicks with bleeding eyes from a combined magpie and currawong attack. The wild birds were able to chase the chicks into huddling against the bird mesh; then I presume they managed to poke their beaks through the bird wire. Two magpies and a huge currawong fled when I walked into the shed.
The poor malay chick copped it worst, but here's a shot of her 2 days later (I won't scare you with images of the fresh wounds). You can see the damage to the lower eyelids, already healing. Her eyesight is none the worse for wear.
Needless to say, the brooder is now fully mouse meshed and the top is partially covered in a sack to limit aerial access.
What this goes to show is that a brooder that works well in one environment may not work so well in another. We've only just moved here; up north, we had no problem with crows, magpies or currawongs, and the dog took care of most non-aerial predators. Down here (Central Coast NSW) the creatures that want to eat chicks are all a lot wilier, being well familiar with people and their antics.
No doubt I'll be modifying my set up even more as I go... But for now, she's safe. :-)