Then place the 17.5"x15.5" speaker board on top of the support slat. To make sure the cabinet walls end up being flush with each other, take two pieces of scrap Baltic birch and place them on the sides while a friend holds the speaker mount and side piece together (Fig. 10). Let the glue dry enough that you can carefully lay the front wall on a flat surface with the speaker mount pointing up. Then let the glue dry on this perpendicular construction.
Step 9. Put a stripe of glue on the top of the speaker board support slat on your second 24"x18" piece. This becomes a side wall. Repeat this step for the third 24"x18" piece—the other side wall. While the glue is still wet on these two support slats, return to the front wall with its speaker board, and run a stripe of glue along the interior where the edge of each side wall will join the front (Fig. 11).
Now clamp the side walls to the main wall, so you have three 24"x18" pieces joined together, as in Fig. 12. The speaker mount is glued to three support slats on the front and slides, and the front edge of each side is glued to the front wall. Wipe away any excess glue and let the frame dry.
Step 10. Now it’s time to install the support slats for the removable rear panel. Turn the box on its side, measure 1/2" (or the thickness of your wood) in from the edge of the open back, and then glue one of the 15" rear panel slats onto the side wall, as shown in Fig. 13. When the glue is sufficiently dry, turn the box over on its other side and repeat this process, making sure both slats are positioned identically. Let the glue fully dry on these rear panel support slats.
Step 11. Though we won’t attach the removable rear panel or roof quite yet, this is a good time to go over the edges of the roof and box corners with sandpaper or a sander. Stand the box up, press the rear panel up against its support slats, position the 18"x18" roof on top, and smooth out any rough edges (Fig. 14). Do a loose “test fit” to make sure everything squares up. Congrats—we now have a 24" tall box with four sides—the front and sides are permanently glued together and the rear panel is currently unattached.
Step 12. This is a good time to drill pilot holes for the wood screws that will ultimately secure the removable rear panel to the side walls. The screws will run vertically along the very back of the side walls, passing through them to penetrate the left and right edges of the rear panel.
With the box standing up on your workbench, pick a side, and pencil in marks for three screw holes. At 1/4" in from the side’s back edge (or one half the thickness of the rear panel), make three marks at 6" intervals (6", 12", and 18"). Repeat this on the other side, and then drill your pilot holes.
Part 2: Installing the SpeakerStep 13. After the glue holding your box together has dried and cured (this can take anywhere from 24 to 72 hours, depending on your glue), you can now install the speaker. Place your speaker in the hole and screw it into the mount.
Step 14. Once the speaker is screwed in, test it to make sure it works. Using copper alligator clips, attach some speaker wire to the speaker terminals and a mono jack. Grab a speaker cable, and plug in any old guitar amp with a speaker output rated for your speaker, turn on the amp, and strum a few chords. If things sound right, it’s time to move on.