A Box Collider 2D does not align completely with a cube's rotation

I have a cube in between 2 sprites. It is scaling to stretch between them and I'm using Transform.LookAt() to set it's rotation so it is always in the correct position.

I've now added a Box Collider 2D to this, but the rotation of the collider isn't matching the rotation of the cube.

When one sprite moves around the other sprite, the box collider 2D is aligned to the cube for exactly half of the movement. For the other half, the collider has a different rotation. See the gif below (using a clock face as an example, the collider is aligned from 12-6 but misaligned from 6-12):

I've attached a zipped example project so you can see what's going on.

Many thanks in advance.