Step 7: Enclosure
I didn't get around to doing this though. I was working on this project one day (messing with the code) and my Old Man came home early from work! He walked through the door, lunch pail in hand, and saw me... blue faced and trying to fart on my Arduino. I tried playing it off cool, "No Pops, it's not what it looks like... everybody's doing it these days, I swear!" but he wasn't having any of it. He figured out what I was up to and the gig was up.
So I didn't put it in an enclosure, BUT it isn't rocket science. Drill a hole in the front for the IR LED and drill a hole in the top to stick the mesh bit of the gas sensor out of. Everything else goes inside the box.
Hide it between the cushions, aim it at the TV and wait for the person sitting on the couch to fart.
You've done it.