6 month validity is required for Tourist Visas which is what the airlines work on but Visa on Arrival and Visa-exempt entries only require the passport to last longer than the permitted stay. That is the rule but even Embassies give out the wrong information so good luck with finding the official statement.
It's not VoA as that is something completely different. Your visa-exempt entry can be extended for 30 days for 1900 baht and if I was you I would book in a cheap hotel one or two nights before and the hotel will ask for a copy of your passport which they use to register the TM30 online. Tell them you may need a copy (screen shot) of the TM30 registration but if they are uncooperative simply take proof of booking of the hotel you are staying.