I've done the DIY filter thing before, but now-a-days i prefer to buy 1 of
these Can33 Filters, i do so every year. This way i know the proper cfm rating for the carbon, if you blow too much air too fast through any carbon filter its useless. I made my own flange for it, and just used this
84cfm pc fan. The fan under rates the cfm flow for the filter, so i know it will last at least as long as they say it will (12-18 months) and going under also assures the air being blown out doesn't smell at all. Well worth the $120 investment for everything IMO. I mean that's $10 a month to not get robbed, haha..
I don't know what kind of space you're growing in, but I'm pretty sure it doesn't even have to be air tight. In theory i believe, as long as you have a negative vacuum, you can grow in a room sized block of swiss cheese, lol. As long as you're pumping air out of the room through a carbon filter, air will come through the cracks and holes in your grow space (which is good because you want to exchange c02), and as long as air is coming into the grow room through cracks none is getting out, so no ones smelling it. I did a smoke test at the bottom of the doors to my grow room, got me real high too laying on the floor taking way more hits then i normally would to get high, but smoke gets sucked into the room from under the door, not blown away, so i think i must be good.
EDIT: just did the math for my room even though i don't think its anything anyone needs to worry about, and i was right, lol... my little ass 84cfm pc fan, blowing through a Can33 carbon filter, in my 11'x12'x10' room, exchanges the whole rooms air volume every 15.4 min. Do you know how long it would take a room that sized packed full of plants to use up even 100ppm of c02? Waaaaaay longer them 15 min that's for sure.
The c02 ppm meter i had,.. when i would breath on it i could make it read 2000ppm in seconds,.. once i had it in my big bedroom, and me and my girl fucked, and we set off the c02 alarm it had for 2500ppm, lol ...c02 is everywhere, just breath on your bitches and they'll get you higher!