To be absolutely safe, you should book a quick flight to a nearby country like Malaysia, Vietnam, or Cambodia etc. Heck, take the 3 day excursion. It will save you from having to go get an extension.
Some people use a service to book a "throwaway" ticket.
Some people will press their luck, and if their airline denies them boarding, they will quickly go online and book a flight. (Luck of the draw on the prices you will find). Thai immigration is not as easily satisfied I don't think.