How to fill void before plastering

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?

Cut bricks lentgth wise

Foam

Build a wardrobe over it.