Apostrophy for possessive is OK. Iirc, it's just uncommon on "it's" solely to differentiate between "it is". I know for a fact this is what I was taught in college and still have the English book. Some teachers and books written by those teachers pretend there never was a hard rule for possessive apostrophies.

For example, the AP styling guide says do not add an extra 's' for singular possessive when the word already ends in s or z, but traditional English rules say do it.