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Hobby Knife Cap
It's a knife cap. I printed it in PLA with a 0.4mm nozzle, and used a heat gun to form it to the knife slightly, for a better fit. You may need to scale it up or down slightly, depending on the particular knife. Also, note that PLA tends to creep and loosen with time, so ABS or Nylon might be better material options.
Update: I have now printed this in Taulman Bridge Nylon, which seems absolutely perfect for this model.