I get the concept, and I've noticed that I liked playing my thicker-necked Kays better than the thin one until I replaced the thin fingerboard. All good. The question that begs asking, though, is why a thin-necked bass is unplayable, yet I hear that many adults, even master musicians, play the cello all day long.
The cello requires less force to depress the strings and the traditional arco sitting position takes the whole balance the bass on the peg against your body issue out of the technical equation. I would say that anyone who really feels like a child's instrument is the best thing for them would be well advised to sit. That would make what will always be a problematic proposition less complicated.
If a small instrument worked acceptably well don't you think more bass players would be using them? Double bass players are as lazy as the next guy and there are reasons we put up the size of the thing....there is no better mouse trap. Due to chronic tendon issues I'm pretty much done playing the DB and I've explored "upright-like" options to somewhat fill the void it will leave. Based on ten years with the Azola Deco pictured below as my main instrument a high quality fretless ABG is something that can be acceptable sonically and visually to many people looking for a double bass player. For several years I was very successful doing acoustic après ski bar gigs with this bass. You do have to be prepared to loose gigs with people for whom the looks are more important than the sound like with the rockabilly crowd but this bass satisfied many acoustic diehards who would never have hired a guy with a "Fender".
ABGs like this that look and sound the part are expensive and hard to find though. Steve Azola is no longer building and the best of them right now is probably the Ribbecke Halfling. I think the regular "production" model (still built one at a time to order) is around $8500. This one in the link for $10K is off Tom's bench and about half what one built by Tom himself would normally cost (without the two year+ wait). It's a very good price for what no doubt is a magnificent bass.
Another option might be the 34" scale NS Design Omni Bass. They're kinda weird to play but they do have a neck profile that allows good left hand technique.
I guess after hearing this question several times over the years I'd say to a bass guitarist looking for a cheap, easy to play solution to getting upright gigs without raising any eyebrows is that there isn't one. It's a difficult expensive instrument not to be taken casually in terms of time, effort and money if you have any real intention of meeting the expectations people hiring you to play it will have.