Rode today with a Revelate Designs Terrapin bag. The holster part stays attached to the saddle and seatpost, and the bag slides in and out. Previously only used this for bikepacking style touring, but no reason not to use for everyday commuting.

https://www.revelatedesigns.com/inde...-bags/Terrapin

Works great as a summer setup, where there is no need for extra capacity to carry morning ride clothes home in the evening. Didn't pack a lunch today, but the bag is only half full, so plenty of room to spare. Nice to remove the rack for a change and go with a lighter and more aero setup. The bag is a waterproof drybag and fairly heavy duty. Backpacks are out of the question for me, especially in the summer.