I've done 2 15 night quarantines, Thailand and Vietnam, never once could go outside, one no open windows. Rough without an open window. I would've killed to set foot ouside for an hour!
Are you sure they aren't referring to the STV visa? That requires booking for your whole stay, but it's usually 90 days initially. If you absolutely must book, which is very weird for a regular tourist visa, there are hostels and really cheap dorm rooms that you can cancel, people have used those kinds of places before. Check Agoda/Booking.com etc.
Just make sure it covers asymptomatic hospitalization, in case you test positive when you arrive. It's mandatory hospitalization (or in a hospitel), even without symptoms, and most international insurance doesn't pay for it. Thai policies do cover it. It doesn't happen to people often, but it does happen and people have had to pay huge bills out of pocket :-(