I've just started my Cafe Build/Restoration and have been toying with the idea of clipons or clubmans. A pair of GB500 clip ons came up cheap so I bought them - thinking if they looked rubbish I could sell them on again. I think they look pretty good, I was thinking of how to mount them - I have seen a few people dropping the forks through the top Yolk and mounting them on top I was concerned the Forks will bottom out, I don't think it looks right either. I quite like the original headlight ears - good place for indicators and they space out the headlight well, with the build costs slowly mounting it was nice to re-use something.
Originally I was going to just cut out a notch so that the clip ons could fit between the ears (shaded on the photo's), but on having a closer look at the ears I could see that the bottom cup was seperate to the top, its held in by three spot welds. If you cut off the bottom cup just above where it meets the shaft of the ears you can see a gap - I used a sharp screwdriver, pliers and a Dremel grinder to remove the spot welds, cut the shaft on the ears to the right size, then pressed the indicator cups back onto the shaft - it doesn't seem to need spot welds to hold them in - they are a tight fit anyway. If they start to move I might put some Expoxy resin to hold them.
Not sure if all the CB's have the same style ears - mine is a F1.