I have a rack.

-Bottom boat sits on cradles facing forward.

-Middle boat sits on the rack on cradles.

-Top boat is flipped upside down and sits on a the middle boat beam to beam, also facing backwards. I make a simple frame out of 2x4's then pad the beams with some thin neoprene type material.

So far I've piled on about 4k miles with a triple stack like that and have had zero issues.

I'm boatless.