We've used these kind for years, and we've no problems what so ever.

https://www.bettymills.com/heel-protector-pad-one-size-fits-most-natural-white-2ea-pk-503020

This is the type of heel protector used in hospitals and rehab to prevent pressure injuries and can be used at home. It feels like a pillow and stays in place to keep heels elevated: https://www.bettymills.com/heel-protector-boot-prevalon-heel-protector-i-one-size-fits-most-black-blue-mon75033001