Hmm that's actually a tough issue given that the default of sorting at the Y position is not gonna cut it.
Perhaps the easiest solution for the fences specifically would be to split it up into individual sprites (and use hierarchies to stich them back together), so that sorting by Y position would approximate more closely to what you'd expect.
I can imagine there could be a more involved solution where you figure out the draw order based on some hidden top-down representation of the scene.
Or it's possible that you utilze the 3D capabilities of Construct. If you imagine the scene as isometric 3D, the fence would be on the side of the cube object, and the sorting is handled by the 3D magic.