Fred Perrin has a similar design (some other small weapons ideas there too): http://www.donrearic.com/perrin.html
but I’ve also seen one that was just a long flat tube of leather that would be easy to make.
I’ve also seen a leather bookmark at Chapters book store that was stitched leather with a heavy metal disk stitched in on both ends with the centre of the bookmark being slightly narrow. This looked quite a bit like a sap though and if you had it on you without a book it would be hard to explain any secondary use to a cop. The coin ones would be easier to explain away.
i was looking for a thing like that, but i didnt wanted one that looks like one. because this might cause problems. so i bought a adult pencil case from jansport (model jtms9) and i think it will do the job quite good and it does not even far looks like a sap.