What should I do to prove I have a place to stay in Thailand after quarantine?

The request is very clear for grant of visa. Another option is if you know someone who lives there, they can provide a letter of invitation and copy of their house registration, ID card and passport if they have one.

The applicant is required to provide one of the following evidence of payment:

9.1 A fully paid hotel reservation for your stay in Thailand issued by the hotel showing the duration of stay, and hotel’s contact details and tax ID number;

9.2 A fully paid residential lease in Thailand issued by the lessor showing the duration of stay, and the lessor’s contact details and tax ID number;

9.3 A hire-purchase or mortgage agreement of immovable property;

9.4 Condominium title deed;

Define the word “prove.”

Book a hotel for at least 7 days.

Had to book one myself today as they wouldn't let me go through immigration without address in Thailand after ASQ was over.

Just book somewhere in bangkok for a couple of days after quarantine. If you go on booking.com a lot dont need credit card booking. Just cancel when you arrive, or stay couple of days its easy to visit tourist spots at the moment

If you stay in hotels, it's up to the hotel to carry out this chore and produce the necessary to immigration

what a stupid requirement

Very good question.