I'm kinda tossed about what to get. I'm not sure if I like the look/feel of a round ball knob?
For a "test," spend $15 or so on an "8-Ball" from a parts place or truck stop. They are very common.
While not a perfect fit (I used one of the smaller than correct insert and drilled/threaded it to fit correctly and tightly), it will get you the idea. I LOVE the feel of a round knob myself.