The phrase "set fast" is often used to describe something that is firmly attached or fixed in place. However, there are various other synonyms that can be used in place of "set fast", depending on the context in which it is being used. Some other synonyms could include "securely fastened", "fixed firmly", "anchored", "locked in place", "stuck", "secured", and "immovable". These synonyms might be used when describing an object that is difficult to move due to its secure position or when referring to a situation where something is firmly established and difficult to change.