I have one of these.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Mounts firm so it doesn't wiggle back and forth as I ride. It's totally out of the way. And it holds way more than you'd think.

I keep a spare tube, tool kit, wallet, keys, small first aid kit, and it still has room to put a couple of granola bars in there.