I suppose this isn't helpful but I don't think the outdoors is the place to sharpen. If you simply have to have something along for the task you could get a small DMT diamond plate but like replacing a head gasket on a vehicle or performing surgery, the field is not an ideal location for sharpening. The best equipment for the task is not all that portable or convenient, IMOHO.
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman