The Alpkit Stem Cell is nice. I usually prefer it to a top tube bag, as its out of the way of my knees, but still easy to reach.
You can squeeze quite a bit of stuff in. I can fit a phone or camera, plus a thin jacket or arm warmers, plus a cereal bar etc.
Yes, not exactly cheap, but seems decent quality, and made in the UK. Maybe better value if you buy the bundle with 2 Stem Cells plus a top tube bag. Then you can put one each bike, or use them all together if you want.