I have a phone mount extending forward from the stem and by pure luck it's all my bag needs for it to lay across, strapped to the top of the drop bars with a velcro loop.

Well, it's home-made and likely much sturdier than available phone mounts but it doesn't actually need the phone mounting platform to use for the bag. Just a strip of hardwood strapped under the stem, covered with fake carbon fiber vinyl so as to not offend anyone's sensibilities, is all you need for a bag mount. Some kind of strap and clasp arrangement at the top and bottom of the bag and you're in business. I extended it to one side with about four inches of plastic to cover the headlight, which is mounted under the bar, to keep the bag from drooping down over it.