Depending on the site - you could ? leave the cornice as a loose item. You’d
need to provide bearers at the back of the cornice, and corner blocks to support
he corner - in cross section, imagine a T on its side, where the vertical bar is
actually the cornice leaning out, and the horizontal bar sits atop the casework.
A full width bar runs at the back between the ‘free’ end of the returns. If
desired, you can screw the bars/supports down to the case at the centre points,
but that would be overkill.
All manner of bracing etc can be provided, your only issue being to ensure that
it is a loose enough drop fit that the casework expansion is not able to stretch
the joints apart.