I cut the sides to fit onto the wall and screwed wedges to the bottom to keep the box from sagging. I didn't measure the wedges as well as I could have and the box torqued a bit when I filled it.

I placed the box on the wall so that more of it hangs over the roof than sticks out out of the back. Almost all of the weight is directly on the brick wall, but having it be a little front weighted means it will tip onto the roof and not fall off the roof if it should happen to come loose.

Using a staple gun I lined the box with plastic, and drilled 4 rows of 1/2" holes every foot or so, and holes in each corner. Then I put screen over the bottom to keep the dirt from running out. This has worked really well, we've gotten some huge thunderstorms, and while the boxes will fill with water they'll drain out in 15 to 20 minutes.

One detail I added to the boxes was drilling 1/4" holes along the top edges for attaching string. I put a clothesline 4' or so over the boxes, and I just tie the string to one hole, toss it over the clothesline, and tie it to the opposite hole. This is for climbing plants (like peas) to use

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