If you aren't happy with an under the seat tool bag, something like this (not necessarily this specific one) but something like this is useful.
https://www.amazon.com/eoGEAR-Medium-Century-Bag-Gray/dp/B00NMRZ4IG/ref=sr_1_41?ie=UTF8&qid=1529468104&sr=8-41&keywords=cycle+bag
Be careful of the ones that hang over the frame. they get in the wa of your knees if you stand up to climb. This the is narrow and doesn't get in the way.
Personally I use an under the seat back containing my spare tube, CO2 cylinder and tire levers plus cel phone.
Alternately I use a quad lock cel phone case and holder (the same setup I used all the way back form Alaska on my motorcycle, so yes, I trust them).