I'd probably start by turning the headstall around first.
Then I'd get an extended Chicago screw and a scrap of thin latigo leather about half inch wide. Take a wrap around headstall, make a mark for a hole thru the end of the latigo and where the screw hole needs to be in the tag end to hold the headstall tight. Then, make a loop just big enough for the fiador, to hold it snug, and back to the hole, cut to length. So, you end up with one piece of leather, a hole on each end of it, and a hole in the middle. Kind of like a figure eight if you will. It will hold it tight enough not to slip if done correctly, and looks fine enough.