What I want to do

Reduce reflexions inside the papercraft foldable spectrometer.

My attempt and results

Following one of the upgrade options suggested by @warren for the Desktop Spectrometer 3.0
I painted the inside of the spectrometer using black ink. A standard black papercraft has no more than 80% of grey so using the black ink (I've used Staedtler mate ink) darkens the color very much.
I also taped a papercraft "wall" inside the spectrometer with a hole on it to reduce posible reflexions too as indicated in warren's post.

Questions and next steps

The papercraft I use generates some grey areas in the black part of the images and by now, I'm not having this artifact using the painted one. I'm not sure how much of the darkness increase is due to the paint or the "holed wall" inside.
Anyway I think It can be possible to darken the spectrometer with paint and some walls inside.
I'll build another one with two or three "walls" and post some pictures this days comparing them.