Advice on repairing mock Tudor

[Wid]

Hi ,new to this forum and a complete amateur so be gentle !I am trying to repair the gap in the photos ,have cut back to none rotten wood and applied hardener,can’t use filler yet as at the back of the 2 holes it’s just the damp proof material in the loft,the ply has been painted with dulux weathershield undercoat/ topcoat
Was thinking of cutting it level and somehow trying to glue pieces of wood across the hole ,then fill.
Any help much appreciated