I have been lucky to score a cabana as a silver, gold and platinum member!
You never know, keep checking daily on the website, if you have no luck, once you board you can go to guest services and ask to have your name placed on a waiting list and if any become available they will contact you.
I will tell you the only time I was not able to get one was when we sailed the Fantasy in 2014 on the cruise happening during President's Day week.
The ship was packed and we had no luck.
Last week we were on The Magic with our grandkids, we booked 45 days before and of course they were all gone at that point.
I went to Guest Services as soon as we boarded, they called us before we even sailed off (CC day was the following day) and had one for us on Family Beach. There were actually 2 others that went unrented that day.
So you never know, you may get lucky!!!
Good Luck and have fun!