Does it have to be natively USB-powered, or are you OK with using a Ripcord or Birdcord? That opens up a lot of additional options, like any of the MakeNoise 0-Coast/0-Ctrl/Strega trio, or Cre8Audio East Pest/West Pest.
Anyway, I hesitate a little to recommend this one, because you’ll have a hard time finding one, but my own favorite little portable semi-modular is The Human Comparator’s The Recursive Machine. Only semi-modular I have ever found that is normalled for crazy feedback. Yes, you can break the normals and use it like a more “normal”-sounding synth, but I love that it comes ready to do the kinds of drone and feedback stuff I love without a single patch cable. It was DIY-only, and they’re all sold out, so you’d have to find one on the used market, but it’s worth hunting down. Built-in (crappy but usable) speaker, carrying handle. Runs on 4 AAs, but can either be converted to a USB jack fairly easily by someone with DIY knowhow, or just throw MyVolts ReVolt “batteries” in and convert it to USB that way (that’s what I did).
Hainbach had fun with it: