You will want to do just 1/4 of it, and then mirror it twice. make sure each mating face is exactly tangent. I have uploaded a copy where I have done this:
Also, you need to make sure your sketch endpoints are exactly coincedent and/or pierce. They have to touch exactly.
Thanks for the help guys. After fooling around with it all day yesterday and cutting some test pieces I finally got the shape approved.
Charles, thanks for showing that file to me. BIG HELP!!!! I knew I was suppose to be doing it in 1/4th's or something, I just didn't know how to go about.