The completed tunnel entrance has now been painted - I used various acrylic paints to copy the well-worn and aged wood effect I had seen on the internet.
The internal shuttering was painted in the same way. Above is the internal (right) edge and below is the external (left) edge. Both shown with a 1:27.7 or 11mm = 1 foot scale figure.
The final two images show the entrance and shuttering placed on the kitchen table and outside patio paving to show the finish I was looking for.