Try sawdust and flat black paint.
- Take out optics
- Apply generous coat of flat black to the inside
- Before it dries, throw in a handful of sawdust and roll the tube round and round to spread
- When dry tap out any loose sawdust that has not stuck to the paint.
- Recoat with flat black, using a sponge.
- Replace optics
Guaranteed to have no internal reflections