Is that not what you are essentially doing here? (in photo)
Originally Posted by Trond
I can state with almost certainty that no self respecting engineer would design a collar for an enlarging lens. Your assembly can not be correct.
Does your cone mount from the narrow end or the wide end?
Every cone-lens extender I have used mounted to the enlarger at the wide end and the lens mounted to the cone at the narrow end (rear element towards the negative). It was an efficient way of putting some distance between the lens and negative without over-extending the bellows.
I think your cone is designed to mount a lens in the neighborhood of 360mm, minimizing bellows draw.
The wide flange of your cone probably fits the same slot as your three lens turret. Use one or the other?