Hi when fitting a new smaller width fire door into an existing opening 10mm max each side what is the best method to fill a gap where the old door frame went into the plaster sorry for the poor drawing but if red is the wall with a 10mm dip in it where the existing frame went in and blue is the new door frame would a plywood strip (yellow) be ok to fit to the wall (10mm less is width either side of the frame) and then mastic sealed flush to frame