Being first matters :')

I wrote a font for these, which does use the triangle-5 and the vertical layout: https://bobbiec.github.io/cistercian-font.html (recent discussion here: https://news.ycombinator.com/item?id=46939312)

And my associated writeup: https://digitalseams.com/blog/making-a-font-with-9999-ligatu... .

As mentioned in the blog, I think the horizontal layout makes more sense too (in terms of writing order). But just like the triangle-5, the vertical layout is more commonly seen, so that's what I stuck with.

In a Numberphile video [0], Alex Bellos also uses a triangle for 5.

[0]: https://www.youtube.com/watch?v=9p55Qgt7Ciw

It might not be accurate but it does seem like it'd be easy to mistake a 5 and 6 without the triangle. Especially when the characters are being hurriedly written by hand. If I were going to use this system, I'd be sticking with the triangle.

I wish the 6 was a triangle in the other direction instead

It would never have occurred to me that anyone would want to get these into a Unicode standard. This document you linked is excellent, thank you.