Shane,
Don't know what's available in Australia, but just want to let you know there is nothing special about the size hardware cloth that is used. The only requirement is that the bees can't pass thru it. Iddee has taught me that stiffer hardware cloth like #8 has much less issues than say metal window screening as it holds it's shape better and will not collapse and that is all I use now. With that said, window screening does work and I have used it many times, and if it was going to cost me $100 to buy hardware cloth, I would highly consider a cheaper window screen if available. Especially since the trap out is next door and you could keep a close eye on it and address any cone issues in a timely manner. There comes a point where it is cheaper to just buy bees than invest heavily in a trap out.
rob....