@TheFife On the two Great Lakes boats I've been on, it's easy enough to just shove your pony bottle under the bench. Of course, you can only do this with slung ponies. I greatly dislike sharing a bench with a diver who has a pony attached to his main cylinder, as it takes up too much room.
I don't find my tank mounted 19cf takes up much room on the tank rack, and if it's tight I put my second tank next to the pony bottle so I don't take up any other space. I just put the whole rig into the rack slightly turned, it works out just fine.