Voluntary funding is a nice idea. Yet totally out of proportion.
I already mentioned the veryvery mixed motivation of many sponsors.
Look, I got a reasonably wellpaid job. And I was reasonably well paid since I started my professional life some two plus decades ago. But as I have voluntarily funded (i.e. donated to) a lot of projects, I really like, my entire possession is what I have in my room (yes, at the age , of 50, I share a flat with friends) an old motorbike and a battered Audi plus three bitcoins. End of story. I could have done as my colleagues, buying a flat or house somewhere, and saving for my pension, but I chose not to.
So, I perhaps qualify for saying a thing or two about voluntary funding:
Its rare.
Most people have a very mixed motivation.
And voluntary funders, including myself, want to see results as fast as possible. They want something.

Friends of mine are rich. And immensely busy on their own charity project in asia, which has already driven them into a heartattack or two. Literally. But they carry on. And they chose to give UBI to one of their unlikely friends, a nice guy, unemployed and businesswise totally hopeless. And thats fine - members of the top 1% of society voluntarily giving UBI to one person. I do of course respect their decision and motivation.
But I would not do it. Considering the restricted ressources available, donors want to see a leverage.
UBI does not provide it to the extent that many other projects I know do in making a change on this planet.

If the idea of voluntarily funded UBI does not convince people like me and most of my friends, how do you want to get there?