I figured that it would be easier to cover the walls and top of the windows with malimine. This is white coated MDF, which we painted "booties blue". This means what ever happens in the future I can restore the room to its original configuration, without much pain and remedial work. The painted malimine was attached to the walls and window frames using nails from a staple gun.
Today we finished filling the gaps with filler.
I will put Alice Springs up, and it should now be ready for its magazine "photo shoot"