The Undersaddle XL is the larger of the two seat packs in the Castelli race bag collection (images are not to scale) this pack is designed to carry a couple of road tubes a multi-tool and a C02 system (plus an external sleeve for a mini pump) with a little room to spare.
Held on by moulded magic Velcro and with a seat post stabiliser the bags simplicity is also its biggest asset. Water resistant fabric water resistant zips internal mesh pocket and well that's it. Just high quality minimalism.
Please note if you ride in heavy rain with no mudguards water will eventually get in to the bag so put everything in a plastic bag just in case.