Anyone make one? I'm interested in extending a head tube about 25mm. I could have a machinist buddy cut one in a lathe. I don't expect that most frame manufacturers would recommend it, but I don't see any problem with the idea, structurally (under the top cup, the bottom would be another story). Of course it would have to be at least a few mm longer than the part of the headset that normally presses into the frame. Maybe preferable to that many additional spacers under the stem.