I'm looking for another way to say that I doubt if person X is a good cultural fit for a certain club, but it's important for me to imply that the problem is in X and not in the club. Just to get my intention across, I'll say that in some other languages there are expressions like "X is not crafted from the right material" or "X is not crafted from the same material as the club's members". Can you think of anything similar?
One could say X "does not [seem to] fit the mold" [of the typical club member].
Or "X [is not][does not seem to be] a good match with the club's culture"
But your original, with "fit", is fine.
"X is not crafted from the right material"
X doesn't have the right stuff. (American)
For a proper answer, we'd need to have a better sense of the "register" you desire.
'X is not a cultural fit' is fine and sounds much better than your proposed alternative English wise.
Stating X is not a fit is active voice, asserting your intention and authority in the matter.
Saying "I'm not sure if X is ... " is much more passive. It sounds like you aren't sure enough of yourself to make a declarative statement.
There is "not one of us." It is the title of a song in the musical "The Lion King," http://www.metrolyrics.com/not-one-of-us-lyrics-lion-king.html