This topic has come up here several times before. There are many helpful answers you can see by searching "defensive publication" on Ask Patents. Also, it is not expensive, but to publish on ip.com you still need to buy "vouchers" from them, I believe.
Some others of the defensive publication sites specialize. For example, OIN (Open Invention Network) hashad a site www.defensivepublications.org That iswww.defensivepublications.org at one time that was related to OIN's mission to keep Linux open. Their site's FAQs emphasize that a publication must be enabling to act as prior artThis seems to a patent application.now be gone (March 2021)