I'm looking into whether I can get made a Subal zoom ring (to use with Type 4 ports) for a Nikkor 8-15mm lens and the Kenko 1.4TC. Although Subal has a zoom ring for the Nikkor 8-15, they do not currently have a zoom ring for the 8-15 + Kenko 1.4TC.
However if enough people are interested......
If this would be of interest to you, could you let me know? I understand price would be broadly the same as the current Subal zoom rings although maybe slightly more given the need for extended length.