This is what a slatted rack looks like:
http://www.brushymountainbeefarm.com/8-Frame-ASSEMBLED-Slatted-Rack/productinfo/8672A/
You put it on top of your bottom board, and underneath the lowest brood box. They are used to help increase hive ventilation, and give bees some “hangout” space inside the hive but away from the frames. There is also an opinion that they reduce the draft on the lowest part of the bottom brood frames, enabling the queen to lay much lower in the frames without the brood getting chilled. I believe the concept was invented by the famous beekeeper, C C Miller.
If you can’t buy them in Australia, Rusty Burlew has a design on her website for a 10-frame version. If you are handy with wood, I guess you could just adjust the dimensions slightly for an 8-frame Flow hive.