So I jump in the ute, wander around to that area and find the "box" which turns out to be an old entertainment cabinet that someone is throwing out... total junk, but it has some good bits of chipboard on it that we can use in other projects so we toss it in the back of the ute. And then we go for a drive around and bam... we spot them.
A split wardrobe, about 1800mm high by roughly 800mm wide and about half a metre deep. 2 halves... perfect for making 3 breeding boxes in each half.
So once I get the chance to measure them up properly, 2x3 stack breeding boxes incoming

Now thats a lucky find.