The Castelli Undersaddle XL is a spacious seat pack designed for cyclists. It can comfortably hold essential items such as a couple of road tubes, a multi-tool, and a CO2 system, with an additional external sleeve for a mini pump. The bag is constructed with water and stain-resistant materials and features water-resistant zippers and construction for durability. It attaches securely using moulded magic Velcro straps and a seat post stabiliser for simplicity and effectiveness. Key features include adjustable hook and loop attachment straps, an internal mesh pocket, and a minimalist design focusing on high quality. While the materials offer significant resistance, prolonged exposure to heavy rain without mudguards may eventually allow moisture ingress; therefore, it is advisable to protect sensitive items with a plastic bag.