I hacked my old Sleep Number queen-size into a king-size by converting the air mattresses into two twins. I used twin mattress covers to pull it off. However, after +5 years, I need a new one. This time, I'm replacing it with a better "all cotton" mattress cover. Copy and paste this link to the twin (extra long) cotton mattress cover that I used: amzn.to/49glvnW