For years DIY-minded riders have bodged-up an easily carried container for tools and other on-the-road necessities by hacking up an old water bottle - just cut off the top, stuff your gear inside with a bag or rag to hold it all together and throw a couple of elastic bands round the top to keep everything down. It's not pretty, but it works.
Cage Rocket is a rather more elegant incarnation of this idea. It's a moulded plastic canister with a hinged lid and a thin foam lining so that the contents don't rattle about too much. It's big advantage over a seat-mounted pouch, according to the folks at Cage Rocket, is that you can get at the contents while riding.
Cage Rocket - a tidy place to store