We live in an old brick building with deep window sills.
We thought a window box herb-garden would be a nice touch for the kitchen.
To simplify the installation, we wedged the box into the opening. That way we didn't have to drill into the plaster and bricks.
Warning: This project is not toddler safe. If a little kid tries to climb this box, it could topple. We put some plants in containers to cut down on weight, but if it were filled with dirt, our box would weigh close to 100 pounds. No matter what, if this box fell on a small child it would be enough weight to kill her or him.
Step 1: Measure twice -- the window opening and then the wood you have lying around
Our window measures about 31-1/2."
the box at it's longest is 30-1/2"
The rest is taken up by the end wedges.
The bill of materials for us:
1x8 for the sides -- 2 pieces about 32" long,
1x8 for the bottom 1 piece about 30" long
3/4 plywood ends -- a little over 7" tall by the same width as the bottom.
Some glue and nails. If you want to splurge, then waterproof glue and galvanized or stainless steel nails cost more but are better.
some blocks 8"x10" x 1-1/2" for the end wedges.
We laid out the sides to taper in about 5/8" on each side, so the top was about an inch and one quarter wider than the bottom."
As for the ends and bottom, the angles are so slight (less than five degree) it wouldn't matter if you cut them straight or at an angle. We happened to have a sliding miter saw, so we cut at an angle. If we only had a circular saw, jigsaw or hand saw, we would have cut those pieces straight.
We banged the box together with some hot dipped galvanized nails so it wouldn't come apart so easy. (We used 2-1/2 inch nails which are over-kill and had to pre-drill our holes so the wood wouldn't split.) Because we have a biscuit jointer lying around, we glued and reinforced the joints to keep everything lined up and together.
Take your time on assembly, but remember that as long as you keep your plants living, you pretty much only have to worry about only one face of the box being pretty since everybody will have their eyes (or noses) on the herbs above.