Today we'll cut the sides of the sound box and make the joints. You'll want to use the 1/2 inch blade on your band saw for clean, straight cuts.
To make the sound box, you need two boards, 4 ft by 3 1/2 inch by 1/2 inch. I get mine at Lowes. From one board, mark and cut a piece 29 3/8 inches long and another piece 12 inches long. Then from the other board, cut one piece 29 3/8 inches long and one piece 3 1/2 inches long.
Lay the box out. It helps if you have one of these cutting mats with a grid on it. I like to make a note on the inside of each board which direction is the top, bottom, front, and back of the box. I also draw a line down the middle of the inside of the top piece and the bottom piece of the box.
Next, take a your pencil and sketch the cuts for the joints. The sides are at slightly less than a 90 degree angle from the bottom of the box, and slightly more than 90 degrees from the top, so you'll need to angle the joints accordingly. The joints should be 1/8 inch deep at the deepest part, and a little less than that at the shallow part.
Raise the guide on your band saw so that it's high enough to get the board in there sideways, and very carefully cut the joints. Then reassemble the box and check to see if the joints are good. On this one the angle got too deep, so I took it back to the band saw and cut a little bit more off the outside edge.
Once you have all the joints cut, measure the box from the top to the bottom and make sure it is nearly 29 3/4 inches. If it is longer, you may need to make your joints a little deeper or the arch and pillar piece won't fit. If it is much shorter you may need to go back to Lowe's and get some more boards and try again.
But if you did it just right, you can slide your assembled box into your arch and pillar piece and stand it up like this: