The glyph outlines origin from Yasna 10 of the manuscript of Tehran University Library, nr. 11263. The scanned manuscript can be viewed under Ave976 on
wwww.avesta-archive.com
In the manuscript long [i] and long [u] do not differ, as well as both short and long [o]. Nevertheless, long [u] and short [o] are mapped in the font. Their outlines follow the script style of the manuscript.
The letters for [θ] and [x] have two different shapes in the manuscript, so I have added one of each...