Each of the four corner caps needs to be L-shaped in cross-section, and now's the time to make them that way. Use a tablesaw to cut a 3/4" x 3/4" notch out of one corner of each 1 1/2" x 1 1/2" cap. This leaves a 3/4" step along both inside edges, enough to accommodate the decorative top and bottom rails you need to install next.
Saw some 2 1/2"-wide stock for your rails, then cross cut them to length so they fit snugly between the vertical corner caps. You'll find it easier to get a good fit if you trim the ends on a 5ยบ angle instead of square. This slight bevel is an old trim-carpenter's trick: it helps wedge the rails in place. At this point, it's a good idea to seal the back, ends and edges of parts such as the top and bottom rails with stain before installation. This helps extend the life of the finish by stopping water from soaking in and getting a foothold in the wood.
Next comes the mitred box-top frame. A chopsaw is an enormous help here. If your lumber is typical of the semi-dry construction-grade stuff carried by most lumberyards, you may find that somewhat wet wood of this sort tends to distort as you cut it. You'll see what I mean if you cut all the pieces correctly, yet the joints still look ragged when they come together. The problem is fuzzy wood fibers are holding the frame members apart just enough to make it look like you don't know what you're doing. Here's the fix: start with a nice sharp blade in your chopsaw, then hold each piece of wood firmly (without allowing it to move at all) as you make two downward chops for each cut. You'll notice that on the second pass, the ragged wood fibres come off, leaving a smooth joint surface that yields a tight result. It's a simple fix, but it works.
Cut and mitre the four top trim pieces, seal the end grain with stain, then fasten the parts together into a frame using #8 x 3 1/2" screws. Three-inch screws are ideal for securing the assembled frame to the top of the box, but don't do it just yet.
Building a Pyramid
The tapered corner posts of the top part of this project are what make it an obelisk, and you need to begin by cutting some high-grade 5/4 decking to width for the four corner posts. As you work, you'll see that there are two kinds of parts here: one pair with pointy top tips, and another pair with blunt tips.
The plans show other minor variations in details between these two versions. You'll find a jigsaw is ideal for trimming the steep, top angle on each post. Clean up the cut edge with a belt sander when you're done.
Fasten the two blunt-tip corner posts together first using three-inch deck screws, then add the other pair to the outside faces. You'll find it easier to fasten these two final parts by screwing from the back face at a slight angle. Once again, join parts together initially with an 18-gauge brad nailer to make the assembly process easier.
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