I love my K'nex and have been on a planter kick lately, so i decided to try and combine the two. :)
Step 1: Green Onion Starters
just buy some green onions and place them in a glass of water. may take a week or two to get nice roots going.
Step 2: Plant Onions
I stuck two in this little plastic pot.
Step 3: Make K'nex Planter
this is the model turned upsidedown, so you can see how to build the base
Step 4: Walls
pretty basic stuff, just make sure to cap off the ends so the four corner rods don't slip out.