I'm still not sure if I'm following the explanation. I don't know if I'm just failing to tell what numbers your illustrations correspond to, at least not by referencing the Wikipedia article you had linked (e.g. there is no representation of a number that has two diagonal lines forming an X).

I visited your second link in the comment above this, but it appears to be a hub page containing multiple books on writing shorthand, and I can't find mention of cistercian notation on that page. If it's in one of the books, I'm not sure where to begin looking.

Here is my attempt to take a stab at using the cistercian system as a tally instead of numerals. Though the representations of 2, 3, and 4 are not consistent with what the article depicts as their standard numerals in isolation, I'm hoping the inclusion of their defining strokes may still be clear enough to interpret.

Where it seems to fall apart is 5, as the only other "opening" for a stroke, without removing any prior ones, is the far-right side. But the location of that stroke would not appear to be touched until you get to the numeral for 6 (visually, 5 is simply the 3-stroke combination of 1 and 4). After filling in that remaining segment, what I'm left with seems to correspond to what you had identified as 9, so I'm guessing it's not right.

Other than the possibility of including an additional + inside the box as you had mentioned, I don't see what else can be done to expand the tally above 5, as that now covers all permutations of the standard stroke positions detailed in the Wikipedia article. But even with the two additional strokes provided by the +, that only takes us up to 7.