BV Bicycle Strap-On Saddle Bag - Waterproof Bike Seat Bag with Reflective Strip for Night Safety | Perfect Size Cycling Storage for Phone, Keys & Tools | Ideal for Road Bikes, Mountain Bikes & Commuting

Very impressed by the size. I ordered the Large. I only just received the bag this week, so I can't say much about durability and water resistance, but it fits entirely under the seat and would likely be protected regardless through light/medium rain.With the extra zipper open, I can fit bike lube, a bike multitool, a patch repair kit, a spare bike tube, a small piece of chain, and still have enough room to fit a second bike tube. It does not fit my bike pump -- make sure you measure your bike pump first! But it would fit a very small bike pump or some CO2 cartridges.