A cold air intake of some kind is what you're thinking. Not sure of what specific one, but any is better than stock. Even a stock Focus intake tube is better than our stock piece. Might check some salvage yards for that.
The throttle body diameter is around 2.75" so if you want to build a cold air intake yourself you'll need at least 2.5" pipe. Regular exhaust pipe would work ok. I used that for my intercooler piping. Here's a close up pic of the TB. See what I mean?
http://www.axs4u.net/home/louiem/update/tbinlet.jpg
The connectors are just regular same diameter radiator hose but for the TB piece, it's a bit more trickier. My dad found a intake piece from a old Subaru at a salavage yard and I just trimmed that a bit and used a pre-mandrel bent exhaust piece for the curve.
