How to filter outside air?

[SSquared]

We were on the Texas coast when the springtime pollen really kicked in. My wife was quite affected by it. She got significant relief by wearing her KN95 covid mask, but that's no fun.

We have also been caught in VERY heavy wildfire smoke. So we would like to figure out a way to install a filter on the air coming in through a Fantastic fan. Our first attempt was to cut a one-inch thick pleated furnace filter to size, and tape it over the hole. But our tape job did not hold, and the filter prevents access to the adjustment knobs.

Has anyone found a way to solve this? I've got vague ideas about screwing some C-channel down over the edge trim, and sliding the filter in and out as needed. One hitch is the manual opening knob--it sticks down below the trim. Brainstorming welcome.

(The picture is wildfire ash we found on the belly band of our trailer during the Waterton National Park fire of 2017)

[bweybright]

If you are only talking the ceiling fan then I would suggest 3M connectors and head to your local big box store for what ever filters they have that you like then stick them on with tape. Cut the box away so you can form it and connect to the 3M connectors and pull off and get rid of when you don't need the filtering?

[SYC2Vette]

This does not address filtering air coming in through the roof vents, which was your topic, but does filter air within so long as you have the fan on, so thought it might be helpful. I replaced our two intake vents with the foam "filters" with a single larger return register inlet that accept a HEPA filter. Works well and flow is fine (the restriction was on the oem outlets, not the inlet anyway. The original supplier on Amazon does not seem to list this 6" x 14" size any longer so may take some searching if this is any interest to you.