Ortlieb Mudracer seatpack
Where most seatpacks are fabric-based affairs, the Ortlief Mudracer has a thermoformed semi-hard shell to keep out the elements. If you've got stuff you need to take on a ride, and you need to keep it dry this is the seatpack for you.
The Mudpack's weather protection starts with a full zip protected by a silicon rubber flap which pulls down to keep water and debris out. Inside there's a zippered pocket at the top that's just big enough for a phone, and a soft padded main compartment that'll swallow a spare tube, a basic toolkit and some energy bars.