Like many, I have used a number of different saddle bags to carry a spare tube, multitool etc. on my rides. Each has done the job but the bags tend to wear, sway and aren’t usually very water proof. I have also been on a hunt to shrink the footprint of the bag and each successive choice I have made was smaller than the previous one.
I recently came across a product from a company in New Zealand - The Aeroclam. This is a hard plastic structure that bolts up to the slanted part of the seat rails and sits tight up to the underside of the saddle. The bottom section comes in three different depths allowing you to choose how much space you need. I got all three sizes in my order and could swap them out in seconds. I am using the medium one now but once I go tubeless, I expect I could go down to the smallest one.
I haven’t had it very long but so far I have been very pleased with it.
I thought I would share in case you might be interested. I don’t have any connection with them other than having been a customer myself.