You seem to have a good grasp of what koshimaki represents—it's a long piece of cloth wrapped around the waist, historically worn by women in the Edo(?) period as an undergarment. The term translates literally to "waist roll." Considering the cultural specificity of the term, much like "kimono," it might be best to retain the original Japanese term "koshimaki" in your translation. You could then provide a brief explanation in a footnote or parenthetical, which would help readers understand the term without replacing it with an English equivalent that might not capture its full meaning or cultural nuance.

P/s:
Koshimaki also refers to the belts (obi) wrapped around the lower part of publications like books and mooks (magazine-book hybrids). These belts display information such as titles, prices, and promotional text.