Hello Fellow OPT7-ites,
While there is not a definitive statement that covers all vehicles and installs, the general consensus is that the dust cover can be left off or re-installed but properly ventilated by creating holes large enough to allow sufficient airflow to the OPT7 Fluxbeam LED Conversion Kit fan assembly, see manual.
Airflow is key, as the LED and LED chip set require the ventilation to dissipate heat and ensure proper product life.
Until Next Time,
The OPT7 Customer Experience Team