"Expedia Trick"... Expedia has free cancellation within 24 hours with 100% refund. Tickets to Rangoon, Myamar are like $50 USD from BKK. Book the flight. Print the documents. Cancel the ticket. Boom done.
I've traveled numerous times on a one way only. If you are really worried here is a trick to help... Expedia website allows for 24 hour cancellation on flights with no fee. Book a flight from BKK to Rangoon (currently that ticket is $50 USD). Next print everything out. Then immediately cancel the flight for no fee and get 100% of your money back. Problem solved.