I seem to recall Peter Jenks had this issue when he owned/ran 'Tarka'.
I think he experimented with a clear perspex or black mesh wind break behind the seats at roughly shoulder /head height.
He deduced that the dust ingress was due to the specific aerodynamics at the rear of the car which caused the air (and dirt!) to be
'sucked' back into the cabin.
I think that DaveW has also designed and built a few versions of wind break - might be worth searching the archives....
James
