I don't have the exact specs, but it is an open frame of 3/4" x 3/4" wood stock with nothing in the middle, of course made to the length and width of your hives. I'm not sure whether or not you guys in OZ use Langstroth equipment or not. I saw some of these shims on hives a few years ago, and I don't think they were a completely connected frame, there was about 3" missing from the middle of one side for the entrance.
Basically the shim is like the frame of an inner cover without the tempered hardboard sheet.
I'm not sure if I described that well enough. :roll:
I made some, but I chose to use a more rigid design, by eliminating the gap for the entrance. I just notched a 3/8" deep by 3" wide entrance, that way the frame is more sturdy. I don't like the amount of burr comb built in this gap between hive bodies though. I went back to just using top entrances.