Basicallly its a tapered tube that runs from the back side of your EDF unit to the exhaust portion of your aircraft. Ive made them extend from the back of the exhaus up to an inch, or flush with the opening, or even just inside the opening. Just to experiment with the results.
There are a wide variety of materials that can be used. But from what Ive seen and found out myself is that mylar works very well. But dont want to use the super thin stuff. I believe what I use is .005-.010 transparencie sheets that can be found at your local office supply store.
Now as for the dimensions that are required that is the trick. And it requires a little math to do which I suck at. But there is a program for it.
Click on that link then you'll have to do some poking around. when you find the excell spread sheet input the information it asks of you. This is what I use and it works pretty good.