I have a 97 sport, and the shift knob is round when looking at it from the top, and tear-drop shaped from the side profile - it tapers as it goes down to the lever. Sorry, no picture right now, I'm in the airport.
I have never had to take the cover off of mine. The nut that is inside the knob will just unscrew along with the whole knob. When screwing it back on, it always stops in the exact same spot, perfectly straight when fully tight. I dont know if the nut is in a fixed position to keep it from spinning or what, but I can hear it shaking around in there when I take it off.
What kind of aftermarket shifter did you have? Maybe someone would trade with you?