If you use Nato can's they don't leak fumes and can be kept anywhere. I sometimes have mine on the roof or inside. If it's a Blitz can then they WILL LEAK gas and fumes only too easily and cannot be stored on its side up top or inside and you must use an external carrier of some type. What I've seen are the Blitz carriers bolted onto the rear door above the door handle and the cans strapped onto the carrier.

Also as has been noted if you are lucky enough to find one Confer did offer a blitz carrier that you could bolt just about anywhere (so long as it's out side and right side up). <img src="/forums/images/graemlins/kewl.gif" alt="" />