Wouldn't it be nice to have everything fit absolutely perfect the first try? Maybe someone could come up with a way to make a mold of your unit and send it in to have a device even more custom. Hell, I'd ever go for a professional fitting, somewhat like having a tailor fit for a suit.
Having everyone fit perfectly first time is rather over-optimistic. Some people even need different cages and rings for different weather!
I must say that probably got myself MM jaibird that is realy close to being perfect. If I would order it again it would only be 1/4 inch shorter (only because it could even be less visible as comfort is great even now). Cage diameter and base ring are perfect. But I ordered two base rings. One that should fit me and one that is one size smaller (just in case if I ever need it). But it turned out the smaller one was the one that fit me perfect. Interesting thing is I couldn't wear cb6000 base ring in that size...