Genuine Innovations' Cartridge Cross conveniently stores up to four threaded CO2 cartridges (except for 45g canisters) right on your frame's bottle bosses.
The die-cast aluminium holder tucks away neatly behind a bottle cage and weighs just 22g so it holds a minimal weight and aesthetic penalty, too.
That being said, we're not entirely sure whom this is for unless you typically carry a lot of cartridges with you, as CO2 cartridges are pretty small and easy to tuck into seat bags or hydration packs.
Since the Cartridge Cross doesn't hold the requisite inflator, there's still a critical piece to mistakenly leave at home. Moreover, the cartridges aren't set far enough apart to clear extra-large frame tubes without resorting to additional spacers.
Still, at £14.99 (US$14.99) the Cartridge Cross represents only a minor expenditure relative to the cartridges themselves since it also includes two 16g cans (normally £9/$9.49) and a rubber sleeve to protect your fingers during use so if it's useful to you, great – but if not, it's not exactly an expensive experiment.