External battery power for speedlights

Would be a great idea, except for my camera.... since the vertical grip is integral, that's where the camera's battery goes. What would be nice is a flat pack that attached to the bottom of the camera via the tripod socket, and have the wire from there to the flash. Would be some extra weight, but defintely better than a cord to the belt pack.
That would also make the bottom akwardly long if it were the same size as the grip. If it was the width of the one the man was holding in the video, that would be a really good idea as well.


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