I wanted to build a cyclone for my newly installed Harbor Freight 2 Horse Power Dust Collector. After much research I decided to go with Phil Thiens Cyclone Separator Lid I am very happy with my decision. If you decide to make your own, head over to his site. The instructions are great.
Here is a shot of my lid upside down. I wanted to show the cutout and the elbow.
Again, I am pretty happy with the results.