I actually did some research as to parts cost (from Europe...not from Jimbo, and not as a "mass manufactured hastily-assembled Jimbo 'shiny-thing-to-sell' ", etc.), and have determined that - for it to be worth my while to build one for someone else, I would probably charge $9K - $10K (clean brass, no lacquer or silver, no case, local pick-up).
I'd probably need about $4K in advance, and there could be no complaints about "response/intonation", as the project would be completely experimental.
Of course, I would do my best work, and would base/engineer "how well it would physically fit on to a person" to "how well it fits me" (a c. 6' tall person). Likely, it would feature one or two adjustable pegs - designed to rest on a player's chair seat...(??)
It would feature a 4+2 configuration (as with a classic F tuba) for some possibility of dialing in intonation, and (OK...getting real, here) as bass trombone intonation is considerably more easy for laymen's ears to critique than typical "fluffy"-sounding tuba resonance, it would include at least one thumb-operated slide trigger.
There would be no "F-attachment" in the bell section, as that circuit would be located in the right-hand #4 rotor.
The bore size - through the rotors - would be larger than .562" (typically bass trombone playing slide bore) as rotors typically add a bit of resistance, but the bore size of the circuits would not be tremendously larger than the bore of a typical bass trombone playing slide.
The low-octave (and also double-low octave) fingering pattern (though in B♭) would - again - follow that of a classic 4+2 F tuba set-up, and - no - I would not (were I to actually pursue such a project) be interested in building one with any other configuration, and anyone's else brilliant ideas should best be pursued by someone else.
B♭ - no valves
A - 2
A♭ - 1
G - 1-2
G♭ - 2-3
F - 4
E - 6-4
E♭ - 5-4
D - 5-6-4
D♭ - 6-2-3-4
C - 5-6-3-4
B - 5-1-2-3-4
For those to whom the 4+2 system is foreign, note that - from F down to D, the left hand completely mimics what the right hand does (from B♭ down to G), but - simply - with the 4th valve additionally depressed. Though the term is not widely accepted, I personally refer to this as a "manual compensating system".
I could build one with a lower parts cost using a Jimbo 3+1 compensating system, but the concerns - here - would be stuffiness, build quality, and intonation, so (if such a project were to ever happen at all) I have rejected that option. I have also rejected a 5-valve system, as (though arguably "good enough" for a C contrabass tuba, as the only "screaming-horribly-out-of-tune-crazy-sharp" pitch - with a C contrabass - is "double-low" D, there are still other pitches which - sorry, but... - are just not close enough to the correct tubing lengths (specifically, the four seen above - which involve the 6th valve) for intonation standards required for acceptably-good bass trombone playing.
I'm partial to a particular bass trombone bell section, because (at least, with a playing slide) it's a bass trombone which asks for remarkably little "micro-tuning" of the playing slide (compared to quite a few pricey bass trombones), and - with a valve system - it's obviously of utmost importance that the partials/overtones behave themselves as well as possible. Even with slide trigger, the hope would be that a player/operator would not have be be driven "nuts" via its mandatory constant use.
Here's the thing:
Even around here (in small-town lower-population-density USA) it's pretty easy to find a GOOD bass trombonist who's available to play engagements, and - if they don't live right in the town where the gig is located, they're willing to drive a hundred miles (or more) to play a hundred-buck job...so I really don't think that any tuba/baritone players (even if such an instrument were to turn out quite good, and were it mastered to quite a level) would end up picking up much additional work - via the ownership of such a contraption. Further, to many (most?) patrons/contractors, live music is at least 90% optics, and - even though the bell section would point out just like a bass trombone bell, and WOULD BE a bass trombone bell - the optics obviously wouldn't completely be on-target.
ALL OF THAT HAVING BEEN SAID, via "beginner's luck" (or perhaps "having a good idea which sort of things might INSURE bad luck"...??) I ended up with THE BEST (F) cimbasso I've ever played, and - in particular - in regards to intonation, but "Past performance is no guarantee of future results." WERE such an experiment to "not suck" (though I would not be simultaneously building a duplicate - side-by-side - for myself) I might (??) consider building one for myself in the future.
...and yeah...for ADDITIONAL COST, I could include a playing slide, and possibly even a bolt-on (STANDARD ROTOR) F attachment.