Custom Planter Box

Here is what I did and the final results.

I measured the space out and cut the 3/4 X 4 pieces of wood to fit between the posts. It hovers above the ground between the posts, but there are so many screws staggered in the posts that it is very sturdy. (I built the box and then secured it to the posts.) It has about 8" of planting space depth. I filled it more than halfway with old milk jugs and water bottles. Then I stapled landscape fabric to the inside rim to hold the soil. I used lots of soil conditioner to keep drainage at a maximum because it is in the shade.