Mike himself uses stones to do convex sharpening...
Most, use sandpaper on a mouse pad or leather to do the job.
But to tell the truth, in the field, the knife should never get dull enough to need a real sharpening, if you just strop it before and after use. Pants leg will do, or a piece of leather, or your belt...
For major touch ups, put some polishing compound on the leather...