This is a side view of the finished assembly.
It shows the soil pipe falling below the hood and cut away to clear the back of the planer head. That does, of course, leave a gap.
To fill the gap I cut and shaped a piece of wood, as you do, and screwed and sealed it into the space. It is shaped on the inside to provide a smoother line for the air to flow past.
When the sealant has cured I will test it, maybe tomorrow.