The most likely implementation in the existing US federal government is a negative income tax.
> I definitely would choose not to work or work less if that money came free from government.
You aren't alone, so your $1200 would not go as far in that future as it does now. There would be some inflation (perhaps only localized increased costs to some industries), but I'd suspect that people working less would be a feature, not a bug.
UBI doesn't make people not want to work at all. Work provides income, meaning, and dignity. Some people would move from industries which have lots of crappy manual jobs (like janitorial services) to industries which are currently only hobbies, but with UBI subsidies could be sustainable as a career. I imagine lots more Etsy shops.