What would a justification memo consist of for a period of 22 days of non rated time between NCOERs?

What was the reason? Obviously outside your control.
Underlap in Rater departure and new Rater arrival?
Change of duty?
Change of station (intransit/leave)?
Lack of Rater qualification?
Simple administrative error?
Some sort of training?
Type it up and submit it.

I'm under the impression the request for such a memo is merely to close the holes in the packet for conformity to the regulations that require Eval continuity. Pick the best explanation that doesn't cause waves that is most realistic and put it in a nice little box with a bow on top in the form of a properly formatted MFR. It's the memo that's important, because there is a check list in case there is a lapse of time between evals that requires said memo.

Did your previous NCOER fail to put a non-rated code with inclusive dates prior to submission to HRC? That’s usually how you “Justify” Non-rated time. The narrative usually used is “rated not qualified”, or something to that affect.