I think it will need to be a single piece, although I'm not sure. Essentially the outer faces need to be clean and crisp, the inside of the 'U' doesn't feature cosmetically. I see what you mean by the 'watch strap' approach, unfortunately that wouldn't work with the design of the handles. I've attached a crude picture that might help clarify things:

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The brass part needs to be fully outside of the face of the bag, and the backplate goes behind - only the screws pass through the bag facing, otherwise the leather will be weakened too much.

Would it be technically possible to make these as a single piece do you think?