Anti-reflex paint for lens adapter

[MEPER]

It helped a lot with the flare after the lens tube got its internal painted with matte paint.
If the internal of the tube had circular grooves like we normally see on cylindric tubes the refleksions would be less I think.
Don't know if a cylindric tube would be better than a conical to have less internal refleksions?

[Akira]

I have a similar conical RMS to L39 adapter, and I painted its inside with the blackboard paint.

Unless you need a tiny lens to stick into complicated places to shoot subjects, a cylindrical extension tube could be less reflective, because the part right behind the attached lens is away from its rear optical element.