How would you block the hole

[harlow]

The council boarded where the old water tank was in the airing cupboard but they made a mess of if so I took it all off to redo but what's a better way of redoing it.
When they fitted the architrave they couldn't get the piece with the red line to match with the uprights so they cut the end at 45 degrees and filled the gap with decorators caulk.

 

[foxhole]

What hole are you referring too?

 

[harlow]

What hole are you referring too?

The hole closest to the ceiling with the board in.

 

[foxhole]

Fire resistant plasterboard.

 

[harlow]

Fire resistant plasterboard.

If I was to use plasterboard how would you fit the architrave. If I fit the architrave how it was before then I would have to try and match a piece on the header to hide the joint. Then if I don't do it how it was before then I would have to blend the plasterboard into the wall.

 

[foxhole]

Fit architrave as for rest of doors , fill and sand upper join.