Clarkey,
I wish that I could help you more, but a lot depends on your specific design: the tops and bottoms will vary depending on just how flared you want your shade. I built four sides and then glued them together so there are long glue joints going the length of each side from top to bottom. These form 90 degree angles, of course.
I can send you my Sketchup file if you want….although it's pretty funky because I just used it as a rough sketch to get the proportions to my liking.
Hope that helps a bit….
Ken