Easy, Stupid-cheap carbon filter, can be made to fit any grow box, cabinet, whatever....
#1 Get some kind of lamp shade, my exhaust hole is about 4" big so I used a small lamp shade. cut out the sides, but leave a few places where the shade can remain intact as one whole piece. we're cutting it so the air can be pulled through.

#2 I took the filter media from an old heater filter from my house, most everyone I know has an extra one or an old one they haven't thrown away yet. All you want is the cloth. Although the kind with wire mesh would work well for holding the structure of the filter....anyway...damn ADD lol... get the media and wrap it around your lamp shade, use a rubber band to fasten it to the top. The key is to make folds so that you don't have a bunched up filter, but a clean, and THIN filter for the air to pass through. this will also hold the structure. I left the flaps when I cut so I had somewhere to put the rubber bands and they wouldn't slip down.

#3 Next step, find some card board or plastic lid, I use a giant lid from a rubbermaid bin. Mark a circle about 4" bigger than the diameter of your lamp shade..this will be what hangs it in your grow box. After you cut out the bigger circle, mark around the BOTTOM of your new filter. you want to make a outline of the smaller end so it slides through the hole and is too big to slip all the way through. the wire skeleton of the lamp shade sould hang on that lip of the circle if you cut it right. take your time, tin snips or a soldering iron work best for clean cuts in this plastic stuff.

#4 Once you hang the basket inside the plastic, you're ready to fill her full of activated carbon and throw her in the hole! Now...how simple was that? And I'll bet mine works better than most ones people payed for...

here it is hanging inside my box. the plastic actually makes a big enough lip to block the light from escaping as well, two purposes in one!
BTW...while I'm here...notice what I used for my exhaust fan....a simple, cheap, air purifier...Take the filters out, lay it on it's side, and that thing sucks some serious air...and now with the carbon filter, it smells like a bed of roses in the garage...before, the cheese was stankin to high heaven!
Good luck...and let me know if you need better pictures. this is the first instructions I have ever made haha