It seems a rite of passage here to put one’s hand to a homemade cog tool. Here’s mine, I wanted to replace the 16T threaded cog, and splined cogs are what I can buy. I found an iron plumbing nut, and cleaned it up on my humble Chinese lathe & mill. The interior was bored to 28mm, I trimmed the exterior threads a bit, and then milled four 6m teeth. It worked a charm and I now have a 17T cog, a better mate for my 46T chainring.
That 2.706 ratio converts to 0.755 low and 3.970 high with the Rohloff, a better range than my half-step + granny road bike with its 0.800 low and 3.538 high. I might go even lower, to a 42:17, but will let that come after breaking in the new bike for several hundred miles of local flats and hills.