Method for making 3D shapes (used to make a bike fairing)

Step 9 Build up the supports that will attach the structure to your frame

Here you just add cutout pipe T's (as shown) to your frame on an as-needed basis. You can also add additional supports for things like cyclecomputers, etc if you have underseat steering.
The way that you do this is to first determine your frame size, then get PVC pipe that's big enough to fit it (so if you have a 1.5 inch outside diameter bike tube you want to use a 1.5 inch inside diameter tube). I've found 3/4" or 1" tube to be necessary on fairings (the 1/2" stuff just isn't rigid enough), but as always your mileage may vary.