What you’re building
Two courses of 2x4 on edge make a box with 1½-inch-thick walls and a 7-inch-deep well. The long sides run the full 11 inches; the short ends fit between them. Two slats lie flat on the ground inside the walls to make the bottom, so nothing has to be cut to fit around anything. Inside, it’s 8 by 7 inches and 7 deep: about the size of a two-gallon nursery pot. It’s heavy. That’s a feature on a windy porch.
Pick the board
This uses 89 of the 96 inches, so get a full 8-foot board and make sure it isn’t a precut stud. If it’s going outside for years, pressure-treated works with this plan unchanged; it’s the same size. Untreated SPF with an exterior finish lasts a few seasons.
Cut
Cut the four long sides first, then the four ends, then the two bottom slats. Check the four sides against each other and the four ends against each other; matching lengths make square corners.
Lay the bottom
Set the two bottom slats side by side on your bench, flat. They’ll be inside the walls, so nothing is attached yet. They’re just there to position the first course around.
First course
Stand a long side on edge against one long edge of the slats, and an end piece on edge at each end of it, inside the side. Add the second long side. The ends should be trapped between the sides with the corners flush. Check for square by measuring the diagonals. Glue the four corner joints and drive two 2½-inch screws through each long side into the end of each end piece, countersunk. Eight screws. Then fix the bottom: drive a 2-inch screw horizontally through each long side into the edge of each slat, two per side, so the bottom can’t ride up when you lift the box.
Second course
Build the second course the same way on top of the first, glued and screwed at the corners. Then glue the top edge of the first course, set the second course on it, and drive eight 3-inch screws down through the second course into the first: one near each corner on each side. Clamp the courses together while you drive them.
Finish
Drill four ½-inch drain holes through the bottom slats. Sand the top edges round. Finish the outside with exterior oil or paint, and line the inside with a plastic pot or a bit of landscape fabric so wet soil isn’t sitting on the wood.
If something goes wrong
- Corners aren’t square. One of the end pieces is long. Sand its end, or accept a slightly off box; the plant won’t mind.
- The second course doesn’t sit flat on the first. Sand the high spots on the top edge of the first course before gluing, or run a bead of caulk in the gap after.
- Water pools inside. Add drain holes. Four is a minimum.