Robert Spear makes New Violin Family instruments. It was originally a concept by a maker called Carleen Hutchins with the idea of building a violin family with violin proportions instead of the sizes, shapes, and tunings we have landed on with the modern violin, viola, cello, and bass. [Invalid or Expired Link Removed]
Several of the instruments are designed to be in 5ths like the violin, but he also talks about variations of tunings in 4ths as well, so people in both tunings are happy. I believe that Tomoya Aomori's instrument is one of Robert Spear's basses. He referred to it as a "5bass" with it being a 5 string in 5ths, low C to high e as described.
Tomoya was at one point around 2010-15? producing content about the instrument in this tuning, showing fingerings and variations for orchestral excerpts, writing and arranging a significant amount of music featuring it, and I believe even produced a technique book.
Mario Lamarre makes 5 string basses, often featuring an extension or two. He made a bass intended for this tuning with a removable neck and a second neck with 4 strings. And if you want to look at pretty basses, his work is beautiful.
Contemporary Stringed-Instrument Making
There was some interest in "what does it sound like?" "where does it come from?" type stuff, so I thought that might help.
There are more qualified people to talk about execution, but I would consider priorities of the player too. Are they looking to spend all of their time up there on the top string doing between-a-bass-and-a-cello type stuff, or are they looking for an "extra gear" they can pull out in niche situations where they need it, but they're playing in the bass part of the bass 90% of the time? Ideally the instrument can do everything well, and often players will modify their posture for solo playing vs. orchestral playing, (or "lots of high stuff" vs. "lots of low stuff" in other genres) but see what they intend to do with it.
Where a low 5th string is "out of the way" and you can reach over for it when you need it, a high 5th string always has to be reached over in order to get to the rest of the bass, which kind of puts it "in the way" when not in use. That tends to be the con/downside you hear from the handful of people with 5 string high tuning double basses, but more commonly from extended range electric players. If you're using it regularly, it's an asset. If you're not, then go back to a 4 string, the way our good L(e)ord Fender intended it.
I really enjoyed Tomoya's original music, and the role that instrument can play particularly in smaller ensembles. It really bridges the gap between cello and bass nicely instead of making it seem like there's such a huge divide between them, and the upper register really opens up. It doesn't quite sound "like a bass" up there, but like the instrument that should be between a bass and a cello, and instead of having that high octane-we're at the top of the instrument-feeling you get from playing near the end of the fingerboard, it could breathe a bit more. Have fun with it. I definitely think there's a "missing sibling" in the violin family somewhere down there, and it would be great to see it adopted/adapted more.