This past December, I saw a film called Old Believers and worked on paper to reverse engineer a fibre arts tool I saw used in the film. Here's the update: Mike King of the Spanish Peacock worked from my drawing and produced a replica of the tool.
I have no idea what to call it, so I say belt tool for want of a better name. In the film, weaver Feodora Seledkova uses hers to secure the near end of a warp as she weaves a card-woven (tablet-woven) belt. She tucks the tool into the sash at her waist. The tool acts as a tiny cloth beam.
I thought I'd need a sash like Seledkova's to hold the tool in place at my waist. However, I find I can secure the ends by slotting them through the front belt loops on my jeans, so there's no need to make a sash. I can go straight to weaving.
Bookmarks. I think I just wove some skinny bookmarks.
At first I turned the cards in one direction but that caused the fabric to corkscrew. Reversing the direction of the cards every four turns creates a flat woven band. It's too bad, because the selvedge is not nearly as firm or even with that method.
ETA: I have posted directions on how to make a belt tool here.