For large irregular areas that need a good fit I cut strips of 3mm mdf around 100mm (or 4 inches) wide and scribe the shape of the individual runs,then lay the strips over each other and use hot melt glue to join them where they intersect.
You should end up with a frame of the outside,depending on the final size you may need to glue in some cross pieces just to hold it together while you move it trace on your ply
Hope that makes sense