BHN hospital, one of the best I hear...off Silom Road down Soi convent....I e-mailed them...Their reply:::::: 1). Can I get the RT-PCR test for flying at your hospital? Yes, it costs 6,500 THB.
2). Can I walk in to make test reservation ?? Appointment is required.
3). Can I get the Test Results the NEXT DAY?? The result will be available in 24 hours.
4). Can/will you give me a printed copy of the test results in English as I will need it to show at the Bangkok airport for departure and arrival next country: We will provide a medical certificate in English and laboratory report.

THIS IS EXPENSIVE BUT....GOOD AND RELIABLE....JUST SAYING