Constructed from a sheet of cheap quality sheathing, 3/4" thick, and a standard 4x8'. This ran me about $20, which is really not bad. I had them cut it into 2 foot strips, across the short side. So I ended up with 4 pieces, each 4'x2'. Then, cut them like this:
The handles were a little tricky. We didn't have an oval, so we used a large socket to trace the sides, and left the top and bottom straight. It's not quite as pretty as the drawing, but it works.
Each box has two handled sides which are across from each other, and two of the handle-less sides. We used no fancy joining, just butted the sides together and hammered in a few nails. Four, to be specific. That is, one more than is probably necessary. We nailed all the sides together first, traced the bottom and cut it to fit. (It's not perfectly square.) Then flipped the sides upside down, and nailed on the bottom. All in all, though it may look a bit... rustic... it's sturdy, and should work quite well. Still don't have it planted, as I went on a run and it's HOT! But soon. Tomorrow, hopefully. Until then!