Another filtration option would be to use a clamp and an old tee-shirt. Cut a piece of the tee shirt that you can put over the reflector "intake" and clamp it on there so it filters the intake air before the reflector and fan.
I agree, definately always try to have a filter before the fan to prevent dust build up. It is amazing what you see accumulate in just one month on an air intake filter. It is good to keep the grow as dust free as possible but dust will get in there and build up especially on the vents