What are the immigration requirements for tourists to obtain the 'visa on arrival' exempt stamp in Thailand?

you're mixing up the terms of entry 😕

a Visa On Arrival is a visa SOLD to people from 15 countries when they show up here, it's good for 15 days and it cost 2000baht

I think what you're asking for is the requirements to enter on a free visa exempt entry stamp (30 day).

As long as they don't have an extensive entry/stay history in their passport they shouldn't be asked for anything.

People CAN be asked for the 3 proofs;

- proof of onward travel within the days you get stamped in for

- proof of funds (20K baht or the equivalent in CASH)

- proof of lodging (usually just a few days is sufficient)

However in practicality "real tourists" are rarely asked for any of that, just get the 30 day free entry stamp and go on their way