OK, the size 19 is slightly more marginal than the 20. If you used Nokon, you would have no problem though. Please excuse my polluting your hot thread with an image of my humble scrapheap rescue, but I think it does show that Nokon is so flexible that this arrangement works perfectly, even on such a short run as on the size 16.
I have heard people say they have drilled the dog collar a couple of mm so that the ferrule fits inside, but I haven't bothered and it works so I don't think it necessary. I've heard other people suggest using a plastic ferrule as the dog collar is soft alloy and a metal ferrule will wear it away after n years, but I haven't and I don't see any sign of wear atm.