Very interestiing solution for the front speakers! Does the driver's side one fit in the same way, or is the bonnet / hood release in the way?
Fot others who don't want to cut the dash, there is just enough space under the centre of the dash, in front of the gearstick, to mount a modern stereo without it getting clouted by the gearstick. The same space could also be used instead for a small front mounted sub - see
this thread about installing a high end sound system in a Porsche 914. Given that your car is an estate, mounting the sub there would also keep the loadbay free.