In the past year or so, someone has been selling plastic boxes designed for storing canning jars. Aha--did a search on "canning jar boxes" and here they are:
http://www.jarbox.com/. If you check their "FAQ" page, they show them stacked two high but say how high you stack them has many variables--"your mileage may vary"--in other words. The boxes are designed to nest together when empty for storage, and have indentations in the top to match the protrusions in the bottoms of the boxes to help stabilize stacks. They are pricey but I see that Canning Pantry--http://www.canningpantry.com/jar-box.html--gives a nice discount for purchases of 6 or more.
"A democracy is nothing more than mob rule, where fifty-one percent of the people may take away the rights of the other forty-nine." ~ Thomas Jefferson