How to make builder's opening smaller

[kasser]

I've got an inset fire which is slightly too big for the opening - the seals can't press against the front.
How can I reduce the opening by a couple of cm either side? I think just mortar/fire cement won't be strong enough as the seals will be pressing against it. The mortar may crack too...

 

[ShaunCorbs]

Cement board ?

 

[kasser]

I guess you fix it to the surface with screws and fill any gaps on the front with mortar or fire cement?