Paper of poster thickness will reflect highs down to well under 1kHz. Thinner paper might pass lower freqs, but not much. "The midrange" extends to ~3000Hz. I have made acoustic treatments with brown kraft paper coverings before. I felt that they sounded a tad too forward in the midrange and up. But other experienced audiophiles commented that it was just right. Different taste. I don't know if the forwardness is direct function of the reflectivity, or from the overtones generated when paper membrane vibrates, like in a drumhead. Thin plastic is a much gentler reflector, since it is less resonant than paper, but tough to print a picture on it. I do like the sound of palette wrap plastic as reflective mask fwiw, it is much thinner and nonresonant. Depends how much midrange your room needs. Mine needs a lot, so the paper wasn't so good. If you room is not too lively in the midrange, then paper could be perfect for you.
GIK Acoustics makes custom dye sublimation prints onto acoustically transparent cloth. But it costs a little bit more than paper.
