In the old days you had to stack them because no 2D linear arrangement allowed access to the important ports (ethernet and whatever. Now a days with the pi3 yo

In the old days you had to stack them because no 2D linear arrangement allowed access to the important ports (ethernet and whatever. Now a days with the pi3 you can run your cluster on wifi (kills performance, but performance was never the point anyway...) So with pi3 hardware its possible to get a length of wood 2x4 and screw the pi array to the 2x4. Of course if you want to do stuff with the hardware other than crunch bytes you'll need the stacking anyway to access all the ports. I'm pleased with the article. Just last week I was looking at the GeauxRobot stacking hardware. Its nice hardware, although around twice the cost.