I built zgbot's air filter for my Thing-o-Matic during the machine's build process. I made it to sit on top of the machine, but after about 6 months of being moved around the duct tape started to come loose. Between that and a circular saw I hadn't had a chance to use I decided to make a version out of plywood.
It takes a square cabin air filter about 8"x.75"x1", which I believe is used on Toyota and Lexus cars such as the RX 350. I'm not really sure because I just bought one from an auto parts store that looked like I could build something around it and I never really paid attention. The filter I bought has activated carbon in it to help with the smell.
The air filter goes in the top, and sits on the edges of the bottom half. It's actually a big pain to get in there so if I had it to do over, I'd make one of the side panels on the top section come off (instead of the lid) and slip the filter in that way.
There are 2 80mm fans on top, which make a little more noise (and push more air) than I'd like. The next time I'm doing some internal work, I intend to move them from 12v to 7v so they'll slow down. I've got a simple switch on the line so I can turn them on and off, and I used a barrel plug from RadioShack so I can disconnect the filter and remove it.
The top is a little short, just enough to be able to hang the LCD controls off the front of the machine. The back of the filter sits on the back lip of the ToM. This way I didn't have to put a cutout in for the Z motor. On the sides, I sanded out little channels to be able to fit over the M3 bolts on the top of the machine.
The base section is 3" tall, which was just enough so that the filament drive wouldn't hit the bottom of the filter when at max Z. With the new MK7s, it looks like it could be reduced almost all the way. As it is though, there would be enough room on the front panel of the base section to embed an LCD and control pad, which could look pretty neat.
All measurements are in inches (because I don't seem to own a metric ruler or tape measure), and should more or less correspond with what I built. It's designed for 1/4" plywood. The panels are secured together with brads through the ends (into holes I pre-drilled with a 1/32" or 1/64" bit), and I glued and nailed some 3/4" wood strips in the corners to provide extra support. The top lid is simply screwed on into tapped holes in the extra wood strips. I believe, all together, I used less than half a sheet of plywood.
I've been using it for a few weeks now, and it works quite well. Having the filament feed straight through the top has worked out quite well. It requires punching a hole in the air filter, but it means that it can drop straight down into the extruder.