Many of us have light, hard casts put on a week or so after surgery. In my case, I have one on for 2 weeks, and it does get itchy.
Normal workaround is Benadryl, but that's not my cup of tea. Get yourself a Q-Tip - it's soft and clean - and lightly push down in the cast and scratch. Don't let go of it!
Not perfect, and has some risks, but man it's been a lifesaver for me!