[aerohead]

Would the rule book allow something like an elastic Lycra Spandex panel to enclose this cockpit area ,which Velco's onto your helmet leading edge below the visor ?
In an emergency egress situation it would simply shear away at the velcro attachment points as soon as you moved appreciably.
It WOULD be aeroelastic and probably 'shudder' a bit,but would be better than that enormous void which cannot support any kind of airflow.