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.

It feels like we ought to be able to switch to calling it "sev", where some people just start saying that and other people understand them? I've been playing around it, but every time I do my toddler Nora laughs at me as if I'm being ridiculously over the top: "you said sev!!" Does not bode well for a low-key migration.