They are all just plastic containers with some kind of medium in them that absorbs fuel vapours. The bottom is open so when it is purging fresh air comes in the bottom and picks up the fuel. You can't really have it in the cabin space as there's a risk of vapour coming out the bottom. You want to keep it somewhere dry and out of the dirt also which is why most are in the engine bay. If you need it for compliance you'll probably need a standard one from another car, if it's for your own desire to reduce VOC emissions then there's no reason why you couldn't fasion up something to fit the application.