I used tested designs for my flutes which I found with construction information on Pete Kosel's flute page
I made a G flute as originally designed and a CPVC D fife to which I also made a modified version. The G flute works great as is, but I was having a lot more trouble playing the fife. It was very difficult to reach into the second octave. Part of the problem I found was to do with the thin walls of the 1/2 inch CPVC. To create for thicker walls with the same pipe, I decided to add a lip plate (like those used on metal flutes) made out of a 1/2 inch CPVC connector.
I also made some original flute designs out of the same material as the G flute but higher pitched in the keys of A and C. These are both easier to play in the bottom octave, but take more coaxing to push into the second octave, especially for the C flute.
To make your own designs or to adapt these designs to different PVC tubing or drill sizes, the flutomat
is a handy tool that automatically calculates the hole placement from the tubing specs, drill sizes and key you would like to play in.