First off, I don't know anything about a U-bass. Never even heard of them.
But: If you're looking for the sound of an upright (and I've got two I'll sell you for a
very friendly price if you're interested), I would think you'd want a fretless
acoustic bass.
I've got a no-name (well, it has a name, I just can't remember it at the moment - might be a JB Player) acoustic bass that I bought used. It's filthy, has strings that seem like they're 30 years old and never been cleaned, with action that's almost unplayable. It looks and feels like it's been sitting in barn for decades. But it sounds so much like an upright, I didn't want to mess with any of whatever mojo it's got going for it. Anyway, my point is that the right acoustic with the right strings is probably what you're looking for, albeit the fretless version.
Btw, I had the aforementioned Squire fretless JB, aka the poor man's Jaco. I've heard guys make it sound very Jaco-like, but I don't know what strings they were using. I know that the ones that came on it didn't do the trick. And I don't know why, but I just never could get used to the ebonol board. At any rate, it sure didn't sound like an upright to me. Maybe with the right pickups.
Oh, and I've got a cheapie fretless maple board P, if you're interested.