:Use flat black paint in the tube. Just before it dries, sprinkle oatmeal in the tube and let it dry.

Another way to make what is really microbaffles. Any way to provide surfaces that reflect the light back the way it came rather than further down the optical path works. The problem with microbaffles is that light also diffracts over the edges and thus proceeds down the optical path and the random positioning of the surface also means that there will be surfaces that promote the reflection also. There's a nice instruction on baffles in the ATMPAGE.COM pages. Nice drawings and other things to help explain how to make good baffles.