Pressure treated wood is hard to come by in all the sizes you need, though it is getting better. My boxes are built, to a degree based on the available lumber size. I have found pressure treated 1x8's, so 7 ½ inches has become my largest dimension. This keeps you from having to joint two boards together.
The box will then be 8 ½ inches wide at the top, 6 ½ inches wide at the bottom and roughly 7 inches deep.
Next cut the front, back and bottom boards. The dimensions for the length should be the one determined by your windows. The width of the front board will be the total width of your board, 7 ½ (assuming you are using 1x8's). The back board is cut square 7 inches wide. The bottom board should be cut 5 inches wide with a 20 degree angle to make it meet the front board squarely on the front. (5 inches will be the short dimension).
Building the window boxStart by cutting the end boards.
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To assemble your box, use galvanized exterior wood screws and galvanized 6d finishing nails. The screws should be 1 ½ long. I always countersink the screws and cover them with elmer's wood filler. Drill ¼ inch holes, about an 1/8 in deep, 2 for each the front, bottom and back. Then drill the hole all the way through the end boards with a bit about the size of your screws threads.
Now screw the end boards on to the front, back and bottom. The exterior screws shouldn't need a pilot hole in the front, back and bottom boards, however if they are hard going in, put a little soap on the threads to make them go in easier.
Next nail along the front and back at the bottom into the bottom board fastening it to the front and back boards. Countersink all the nails and then cover all nail holes and screws with the wood putty.
Drill a ¾ inch hole every 6 inches into the bottom board along the center of the board to allow water to drain out.
Finally, prime and then paint the boxes an accent color to compliment your house's colors.
MountingMounting the boxes always seems to present a problem. I hate putting holes in a house. I have found that mounting them from the window's sill works well. But I have also seen them screwed directly into the side of the house where the siding was wood. In any case I will describe the method I have used to hang them from the sill. Buy 3 inch T brackets. Two for each box. These should be painted to match the window sill.
Evenly spaced from either side, bolt them to the window box. Use galvanized or stainless steel bolts/nuts etc. The horizontal top part of the T should be just above the back of the window box. Using stainless steel screws, screw the top of the T into the window sill.
FinallyFor planting it is best to have plenty of peat moss mixed in with the dirt to hold the moisture so you don't need to water as often. I always place a small stone over the holes to keep the dirt from washing through.