How to fill void before plastering

[Ayla84]

We removed the full chimney breast including stack.
For some reason the brick on either side of where the breast in loft was isn't the same depth. If I try to back fill with brick it will be too thick on one side. The gap is about 8 cm so I think too thick for any plastering.
How to best work around this?

 

[Monkey Boy]

Cut bricks lentgth wise

 

[Stewie03]

Foam

 

[Cockney]

Build a wardrobe over it.