If you wanted to "for real" design it, I suppose you could use SolidWorks. But that's a full-blown 3D Cad program meant to design nearly anything, and quite expensive if you purchase it through normal channels. Might be a bit overkill. Good news is that if the cue is done in SW, it converts directly to a CNC.
If you just want the design for looks, I'm sure there are more frontend-friendly programs that are visual in nature, but I don't know what they are. Photoshop maybe.