Blue ebonite pen

I am trying to make a semi hooded section in which the nib unit is slid in from the section thread side.
Any advice how to ensure zero axial rotation movement of nib unit?
Do not want to drill a hole on the section and use a pinching screw.

I started this pen with a slightly variant design of semi hooded nib section, shown in my earlier post.
Finally completed this pen now. Material is blue colour Nicco ebonite and brass for the handmade clip and trim rings.
A Versace size five nib unit and Schmidt k5 converter is used. The cap fits to the barrel by slide fit, no thread used there. The nib unit is fitted with an extra sleeve for axial play control. This my first work with Nicco ebonite, much superior material compared to the lndian ebonite l had used earlier.

Hope you like the design. Let us have your comments.