Mr_Reman wrote:

You CAN screw a Kayfun poly tank on a few turns, But then it jams up a couple of millimetres before you get the O-ring to seal,.

I have a couple of rose clones (tobeco) bought elsewhere. The plastic tank sections that came with them did the same, got tight before the seal was made. This was only on one end, don't remember if it was the top threads or bottom. It wasn't due to the length of the threads, dunno what caused it. I just tightened the hell out of it till the o-ring seated. It works fine, and no reason to remove it unless replacing the plastic anyway.
Which kayfun plastic tank are you working with??? I have a standard kayfun, and its plastic tank section is a little shorter than the rose. Can you please give a sku or let me know what tank you are modifiying?
I don't know why FT can't supply the tanks, people have begged for them a long time, so I guess they'll never be available. It is a damn shame.