Pack out fireplace surround

[Yog1964]

Just bought a resin fireplace surround.

We are looking to have open fire with log basket. The surround when placed against the chimney breast leaves a 4cm gap between the chimney breast the inner of the surround. How can I bridge the gap,? Can I use timber, then plasterboard and plaster? How far away from the flame would this need to be to remove chance of combustion?

Thanks

 

[foxhole]

Gap is to allow room for backplate , what are you using?

 

[Yog1964]

Not having a cast iron fireplace, and gap would still be too big, given its about 40mm. Looking for safe way to make up the gap and then plastering the recess

 

[foxhole]

Metal stud and cement board .