I was in Thailand 7 times last year - every stay around 10-14 days. Entered every single time with a one way ticket. Never had a question from immigration.
One time check in staff at the airport did ask for my return ticket. I told them, I hadn't planned exactly when to leave, and showed them a copy of my work schedule that said, I had to be back for work 2 weeks later. That convinced the check in staff, and she let me check in.
I was travelling visa exempt. If you need the tourist visa, you have to provide a return ticket in the application, I believe. I know some people who have gotten away with just taking a screenshot of a flight booking just before you go to payment site. Of course this won't have a booking reference, but 2 people I know got away with it a couple of times.
Otherwise just book a cheap dummy ticket
Good luck
