Re: a bbox "builder" repo - it's an interesting idea. I could see it going two ways: (a) you want to run a bbox builder yourself, or (b) you want a simple way to specify a bbox so the dataset pack can be produced for you.
I started with the "ship a known-good pack" approach because the build pipeline is the messy part, and I want deployed boxes to stay simple/reproducible.
For your use case, which did you mean - run the build locally, or "draw/paste a bbox and get back a ready-to-run pack"? And would bbox be OK, or do you prefer admin boundaries (country/state/city)?