Is there a practical reason why nobody has made a combination darkbag / focusing cloth?

As far as I can see, it requires two more zippers (with appropriate overlapping of material) in a double dark bag, and your hands can go through the arm holes for fiddling with the loupe.

I might just have to cannibalize my smaller dark bag to try this. It would reduce the crap I carry and simultaneously provide a compact, light tight viewing hood. For reference, I carry a small dark bag with me so that I can change filmholders in the field.

If this has been done, please let me know. I have zero interest in reinventing the wheel.