@Mae *****
Thai Immigration actually is not interested in your return ticket or flight out of Thailand, when you arrive without a visa and enter visa-exempt. Only if your passport shows an intensive travel history to Thailand, you might get pulled aside and asked.
In the majority of all cases, it is the airline which wants to see a proof of travel out of Thailand within the 30 days period you will get stamped in on a visa-exempt entry
You can solve this issue with a cheap “real” one-way flight ticket. i.e. Air Asia from Hat Yai to Kuala Lumpur for like 32.- US Dollar
You can try your luck with one of the “fake” onwardtickets, but be prepared that they might not be accepted
Some people claim that a bus- or train ticket out of Thailand, or a hotel booking in the neighbor country was enough of a proof, but again be prepared that the airline will only accept a “real” flight ticket
There is NO GUARANTEE that your experience will be like some people in the FB groups will tell you, claiming that they have never been asked for an onward travel proof.