I don't like the number seven. All the other single digit numbers are single syllable, and seven has to go and take two. Seventy and seventeen have the same problem.
I think the two main candidates are "sev" (dropping the second syllable) and "sen" (dropping the first coda and second onset). While I find "sen" slightly nicer on the tongue, I think "sev" is more promising because it feels like a better short form: https://www.jefftk.com/p/sev-sevteen-sevty-sevth
@jefftk I think "sen" feels a lot more natural; if I'm counting as fast as I can from one to ten, I'm very likely to blur "seven" into "sen" anyway, and likewise "seventy-one" feels very natural to blur into "senty-one" or "senny-one".
Then again, having said that, I feel like "five six sev eight nine" isn't awkward either. And I might naturally blur "seventy-one" into more like "seven-one" if I'm going fast. Hm.