Hello John Atkins-Lokpathip, International Transit or Transfer Passengers at Suvarnabhumi International Airport (Layover time not exceeding 24 hrs/ check-through luggage only and cannot leave transit area) do not need to apply for the certificate of Entry (COE).
For Non-Thai Passengers required documents
1. Valid Passport
2. Medical certificate with a laboratory result indicating that Covid-19 is not detected, using RT-PCR test, issued no more than 72 hrs before departure or Certificate of Vaccination certifying that holder receive manufacturer's recommended dose(s) of vaccine approved by World Health Organization (WHO) or the Ministry of Public Health of Thailand, no less than 14 days before departure.
3. Travel health insurance covering health care and treatment expenses for Covid-19 disease, of any other guarantee throughout their stay in the Kingdom, with a coverage of no less than 50,000 USD
The passenger must check the required documents with the final destination again.