Do you find it comfortable with the canister lying down? In my pack I find it more comfortable with the canister ( currently Bearvault) standing up. It could be the manufacturers are agreeing with me - or it could be they are just missing it, wouldn't be the first time. The last pack I made, which was intended to be both my ski trip pack and the "papa bear" unit, I made big enough in circumference to take the canister lying down, but then I found that to be uncomfortable so I never pack it that way.
Have you considered making your own?