@Steven ***************
whether you need a return flight or proof of onward travel is more an issue of your air carrier. SOME have the policy that you need to show onward travel within the number of days you're going to get stamped in because you're flying to thailand without a valid visa AND few if any airlines know that the free visa exempt entry stamp went from 30 days to 45 days SO you could be asked by your air carrier to show onward travel within 30 days of your flight here.
CHECK with your air carrier to see
If you manage to board your plane you won't have any issues stamping in here without onward travel as they won't ask you at passport control