I've been thinking about this for some time as well.
I'd like a portable lightweight dive rack that I can take on and off without too much hassle.
I'd also like this to be moveable within the boat so that I can place it where the hell I like....either right near the transom or just behind the back of the seats (to aid with load balancing).
I've toyed with the idea of some high density foam in two rows and lie the tanks down flat in the grooves (if you see what I mean) and fit the bottom of the foam with non-slip conveyor belting.
I would have thought, (but could be wrong until I try it) that the weight of the tanks would be sufficient enough to keep the tanks and foam and conveyor belt in one place without the need for fixing to the deck.
But like all good plans!
Which part of "throw me the mooring line" didn't you understand!?