Every year, I grow a lot of seedlings in 4" pots, and sell them locally.  The way I set them up, I needed a stand that allows me to have about 40 or 50 of them in a large tray, and not tip over when there is wind, and/or the plants grow above a certain height.  This is my answer.  I figured I would design a mini 2-unit plate as well, so they can be used inside as a mini plant propegator