Ventilation Question

I have 2 120mm 72 CFM fans blowing into a 12 x 6 x 6 duct vent with my DIY carbon filter on it. I am about to cut my passive intakes to cover with my filters and was wondering what dimensions I should cut it.

Any suggestions?

cut it so it fits

Have a passive intake slightly larger than the total area of your exhaust, if you have two 120mm exhausts, either 2 130mm intakes or a single 180mm intake should suffice. What sort of filter are you putting over your intakes? That will affect how much air will flow through them and you may have to enlarge accordingly.