I had at one time purchased the small Mattone bag and while I thought it was well made, it was just too large for me (.6L - 2x what I was looking for). I wanted something that would fit completely under my saddle.
I was also looking for a strap to attach a small bag under my saddle that would allow me to open the bag while on the seat. There are options that allow this, but they are all too big (I was looking for .2 - .225L totals size.) or they were the right size, but the hook and loop straps crossed over the zipper and if you undid the straps, the bag would slip out from under the saddle. I decided to repurpose an excellent 76Project.23L bag (and reuse the strap elsewhere) and pair the Hypalon strap to it. The Hypalon still covers the zippers (actually an extra security bonus) but because the BOA cables are still attached, the bag will still hang from the saddle rails even it is fully loosened. The strap also distributes the tightening loads more even, preventing pinching that occurred when I tightened down the hook and loop straps, potentially damaging contents inside.
This setup allows me to carry (2) Tubilito tubes (game changers), (2) tire irons, (2) valve extensions, CO2 inflator and cartridge (could fit another), and a tubeless tire plug kit. I use Specialized' s SWAT Road tool and case mounted to the road water bottle and a the SWAT chain breaker tool in the stem cap, which allows me to have the tools available immediately without digging through a bag (or forgetting it at home).
While overall, I am pleased with this setup, it is not perfect. If any Silca designers are reading, I would definitely buy a .225L bag shaped in a cylinder, 2.25" in diameter at the front, 3.5" diameter in the rear, with a zipper that goes around the rear section. That would allow it to fit in-between the rails of a saddle. If you make it out of Dyneema and tape the seams, you can take my CC now :)