OK. Back to that window box. Let's get the material selection out on the table, so you can get lots of good and bad advice about it from everyone. You'll hear things like:
It's gotta be cypress, for rot resistance.
It's gotta be pressured treated wood, ditto.
It's gotta be cedar, ditto.
It's gotta be lined with copper, or it will rot.
Don't make it from metal, it cooks the plants.
Make it from pine, so it will paint up well.
It's gotta be marine plywood, assembled with epoxy, or it will delaminate.
And so forth.
Well, it could, but doesn't have to be any of those things dear reader. You're gonna get ten, maybe fifteen years, tops, out of your window box, no matter how indestructible it is. Let's keep it simple, and well, picturesque.
We're gonna make the box out of MDO. Medium Density Overlay. Why? Because it's cheap, and easy to work with, and strong, and not too heavy, and it paints up well, and it holds a screw pretty good. MDO is the stuff that road signs were made from, before they were made of steel. It is an exterior plywood, with waterproof glue, and a tough paper face on it, impregnated with waterproofing too. It's a light golden color when you buy it around these parts.
You can make two window boxes out of half a sheet of MDO. A full sheet is 4' x 8.' You could get four out of that, easy. We'll also use some pre-primed #2 pine, 3/4" thick, and 2-1/2" wide, for the bands around the box, to stiffen and adorn it. You'll need about 16 linear feet per box. They call that a 1 by 3. That's called its nominal size, and traces the measurement of the lumber back to before it is dried, and shrinks, and is dressed to its final dimensions. It does seem to the fledgling lumber purchaser that calling something 3/4" x 2-1/2" a 1 by 3 is like calling the small coffee a medium, and figuring no-one will notice. But you are in the lumber yard now, dear reader, and they're not trying to pull a fast one; believe me, they don't feel the need to make any bones about taking the shirt off your back for a strip of wood a bird was chirping in a few weeks ago. It's just one of those interesting and time honored traditions that traces its roots back to Noah, and people who know that sort of thing, know that sort of thing.
Now if you go to the Big Orange Place and ask for MDO, the pleasant teenage girl or boy with the orange smock and braces might mistake it for MDF, which is medium density fiberboard, and entirely the wrong article. MDF is brown talcum powder, mixed with nasty glue, pressed into big rectangles. It's what bad furniture is made from. It lasts approximately ten minutes outdoors, unless it rains, in which case it disintegrates immediately. And it weighs +/- 750 pounds per sheet, or so it seems to if you try to carry it. And it's loaded with formaldehyde from all the glue. Every single thing in IKEA is made out of it. Buy Sippican instead.
Anyway, 1/2" thick MDO is what you want.
Now we're gonna start measuring. You should too. How wide is the window you're adorning? No, no, not the window sash alone. You should include the casings that flank it too. I've got 40" here. That's about average, and not too long for one trough. Really long windowboxes are generally a more difficult proposition, they have a tendency to bow out in the middle of the span from the weight of the wet soil and plants, and require either many partitions along their length, or better yet, you can divide the window box into more than one box. I've made them 10-12 feet long on occasion, but there's a lot more structure in those than we need to deal with here.
(The box is under the three windows ganged together on the right, waiting for spring planting. BTW, the entire "gingerbread" front of that house is MDO, with pine battens on it. It's great stuff.)
Wow, you look fatigued. You go rest. We'll bring out the sawhorses tomorrow.