I've been looking for what you describe since I first got my watch and haven't found one. I would also strongly prefer to just carry one cable and an adapter to use it with my watch. What I ended up doing was buying a small travel charger that has a micro USB port (
this one) and affixing a micro USB to Lightning adapter to it (
this one). It works well enough although I'd definitely love it if Apple made their own as I'd imagine it'd be smaller. I also don't necessarily need a battery built in, just an adapter to use my existing cables.