I'm looking into getting another low "trumpet" in F or Eb built. I'd prefer a leaner, Bb bass trombone sound (partially so I can use it on infrequent bass gigs as I'm having ergonomic problems with big trombones that won't go away) but am open to more of a euphonium sound. I'd like something zingy, light, easy to play, compact, a big sound, and with good intonation with the ability to adjust slides (rings or kickers).

A decent sized valve set from a marching bariphonium in Bb or G might work, possibly a very small bore top action Eb tuba. Eb would be nice as it would most likely be paired with an Eb tuba (so a lighter, bass trombone sound would also work better) and would give me a low Bb and A but F would be lighter.
Anyone got anything like this, got any parts that might work and are not wanted, interested in building it, got any thoughts,...? I would prefer to trade one of my for sale trombones but can also give actual money for it.
One of those Chinese travel tubas in trumpet form with piston valves might be nice.

I sold my rotary bass trumpet as I just couldn't get used to the sideways thing.
Thanks!
Bob