I don't see where it is actually going to get you. There are literally no significant bass guitarists that have gone the route of aping an upright sound.
The best thing for an upright sound is an upright. An EUB can stand in for amplified upright. A bass guitar should speak in it own voice. As far as acoustic jazz, only Steve Swallow been able to make the BG work as far the top end recordings and concerts.
It isn't an interesting problem to solve.
In the long term a context that needs an upright will need and upright. If it is a gig or two, cut the mids, play quieter and listen!
Well, I don't have a fragment of the experience Damon does but I think he's basically got the thing by the right handle here.
1) The only thing that really sounds just like an upright is an upright.
2) You can get kinda sorta close, but you never really get there and by the time you've spent all that time money and effort, you might as well just have brought the upright.
3) There are a lot of things that can fulfill the same FUNCTION in a pinch - after all, that's what the Fender bass was invented FOR - to provide bass players in any genre (except, of course, orchestra) an alternative bass instrument that could do the same things (play two-beat, walk, etc.) but with a whole lot less trouble expense and size.
When you say "Oh, so and so sounds NOTHING AT ALL like an upright", I suspect it's the tyranny of small differences (the princess and the pea syndrome) speaking. "Nothing at all like the upright" would be, in my opinion, reserved for instruments like tuba, contrabassoon, bass saxophone, tubular bells, typmani. Theoretically you could play a gig substituting one of these for the upright, and I assure you it would be way further from the general sound and effect of an upright bass, than an electric bass with the typical minor adjustments.
Personally, I think when I play upright in my more traditional settings in jazz, country, and bluegrass, it sounds better, but when I play electric in those same settings, playing pretty much the exact same things, it sounds a hell of a lot better than no bass at all. If I were to go all Geddy Lee or Entwistle at the bluegrass jam, it'd sound like poo. If I play normal two-beat bass, it sounds good, though not as good as the upright would.
Just this weekend I got a little stink-eye for bringing the electric to the bluegrass jam. I told the stink-eyer, "Look, I just drove 2000 miles with everything I needed for two months, in the bed of a pickup truck. I would have brought the upright if I could, but compared to food, clothing, tools, and water, it just didn't make the cut. You know how I play; I'm not going to turn this into a rock and roll concert." and that was pretty much the end of that.
So if the OP were listening to me, and I don't really know why he would, I'd say - forget trying to find something that sounds just like an upright; you've already got something that sounds just like an upright; it stands seven feet high, has a big wooden box, a fretless fingerboard, and a tall bridge. When you want the upright sound but the load-in is just too ridiculous, say to hell with it and bring something else - and play it as close to how you'd play the upright as you can. It won't sound exactly like the upright, but it'll support the group the same way, it'll be a lot better than no bass at all or bass played with the wrong style, and basically no one but you will even notice.
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