I've heard so many things about the Thomas that it seemed to be the only one to buy. This may be a silly question, but what if I added another filter in the unit, essentially doubling it up?

I tried this and it appears that there is enough clearance in the unit to allow for 2 filters in the unit and the door (with another filter in the door) still closes comfortably.

Of course, that costs money. Am I just better off making a cardboard mask to reduce light output?