Too cool!!!! I gotta have one .....
Now, in all fairness, I'm going to point out that you can pretty much do this mod with just about ANYTHING that will cover the mouth of the carburetor: Altoids boxes, iPod cases, electrical socket boxes ... anything that will cover the carb mouth. (One of my bikes has an air cleaner made from a piston head and the front pinion from an X bike, drilled to match the carb screws.) For the filtering part, you can use sponges, pantyhose, cheesecloth, even a piece of a handkerchief. Window screen is a good idea, and for a seal, I like using one of the red toilet tank gaskets, glued to the can or whatever I happen to be using. The important part is making sure the mouth of the carb is sealed
good, and that you have at least some kind of material over the mouth. I went for the Altoids can because I didn't think the cheesy little plastic air cleaner looked good on the bike, plus it seemed awfully fragile. Plus, being on a ratbike, the look fits with the general "home-brewed" theme of the bike.
Here's the pic you referred to, the famous Altoids can air cleaner. This is my favorite shot, as it shows off most of the main features of the bike, such as the Altoids air cleaner, spraycan muffler, Mongoose footpegs, custom seat, and the dremel wrench killswitch cover:
