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If you have a fan that blows air through your filter from the inside of the enclosure to the outside. It will create an underpressure in the enclosure so air will only be going in to the case via the gap.
(If you seal your enclosure so it's airtight, it will create a vacuum after a while, the fan will be working it's ass off and virtually no air will leave the case).
You can easily test if your fan moves enough air by holding a small piece of paper infront of the gap, if it bends towards the enclosure air goes in via the gap and not out.
What I'm trying to say is; that's good to have a bit of a gap :)
Best of luck!