I would just break down and buy a nice AEM or Steeda ram air intake. These are tuned, length wise, pipe diameter wise, and shape wise to get the most hp gain. You could make your own, but you wouldn't get hp gains like you would with a manufactured intake.
That being said, if you're still intent on making your own, I've seen some creative work with PVC pipe and a filter thrown on the end. You would also have to make provisions for the MAF sensor, which is an odd shape, and would be difficult to fit flush onto the pipe. Any more questions, just ask.