I think you are close enough. You can really chase your own tail trying to get an "upright" sound. Most folks would accept a thumpy, Middy, muted kind of tone. My belief is that most folks who want an upright sound really want the visual appeal of the double bass on the band stand. To me, this is going back about 300 years in technology. I don't see keyboardists hauling an acoustic piano (harpsichord?) around, nor do many guitarists bring along a lute. Leo had a really good idea...go with that, mute it, and you will be fine...