I personally like the sling method (which I've used for anything from an AL19 to an AL80). The nice thing is that you sling it from your BC or harness, so it doesn't matter what tank or tanks you may be diving. Additionally, if you go toward the tech side of diving, you'll most likely be slinging stage bottles sooner or later, so it makes sense just to go that way from the top, eh?
If you're doing some type of diving for which slinging would pose complications, perhaps you may need to look at tank attachment concepts. If that were the case, one idea I've had is to use something like an STA (single-tank adapter). The cam bands secure it to the primary tank, but instead of a backplate, it has a pony attached. It's an idea.