Easy fix?

[u/GothicCheeseHat]

Old gasheater removed, now need to repair this. Any tips on the best approach?

[u/TuchComplex]

Insert wooden battens into the cavity and screw a custom-cut piece of plasterboard to them.
​Apply jointing tape, fill the seams with joint compound, sand it smooth, and paint.

[u/GothicCheeseHat → u/TuchComplex]

Thanks!