I have used a Nashbar Elite handlebar bag on top of a front rack. I removed the original mounting hardware and most of the stiffeners to save a little weight. I used the shoulder strap, wrapped around the stem to hold it in place. It was easy to take on and off because there are dog leash type clips on the strap. I really liked it and found that for me it worked better than any bag designed for using on a rack top.

You mention not wanting to go that large... My take on that is that when I want to go much smaller I'd just forgo the rack and the bag and pack those items elsewhere. In that case you could use something else like maybe one of the little stem and top tube mounted bags. That or I just stick what would go in the small bag in my jersey pockets.

I also have found that I don't mind wearing a tiny backpack like the REI Flash 18 as long as it has no more than a few pounds in it. I used one on the ST for days when I needed to carry extra stuff on long empty stretches, found I liked it and started using it regularly with two or three pounds of stuff in it. It was nice that the stuff in it automatically went with me when off the bike.