Well yes and no.
The front 'cone' solution works most of the time. But I can still get the hotspot sometimes if strong light is at a certain angle to the front of the lens. Usually this would produce a flare or maybe some veiling depending on aperture, right? But with the CO60, you just get the hotspot.
I'm probably going to move to rear extension.
If this lens weren't so darned sharp, I would probably give up on it. But when it works, it works spectacularly well.
There is an interesting analysis by Jim Kasson about the CO60. It is really an f/8 at f/11 and an f5.6 at f/8. I'll go dig out the link.