The phrase "knock props out" implies the destruction or removal of a supporting structure, which may lead to a collapse or failure. Some antonyms for this phrase might include "secure," "support," "fortify," or "reinforce." These terms suggest a strengthening or stabilization of the structures in question, rather than their removal or disassembly. In other words, to "knock props out" is to weaken or undermine a system, while its antonyms imply the creation or enhancement of a stable, reliable framework. Whether discussing a physical structure, a business plan, or a personal relationship, these antonyms emphasize the importance of building and maintaining a solid foundation.