For the top/sides/bottom of the case, the back sits in a rabbet, so when the case is seen from the side, the sides of the back will not be visible. To make the rabbet, I built a "t" shaped fixture. On one side a peice of 1/2 ply is nailed and a 1/4 ply is on the other side. I simply clamped the fixture to my stock and planed away. When I had established about a 1/16" of shoulder I chiseled out the waste until there was about 1/8" left. I then finished up with the rabbet plane.
With the rabbets cut, I could proceed on to the final length dimensioning. First I cut off as little as possible on one end, with a miter box, or freehand. I then shoot the board with my new shooting board(it's hard to think now how I did without it) until the edge is nice and square. I then mark the desired length cut, and shoot, until the exact length is reached. If the waste is too little to cut, but too much to shoot, you can pare away the waste with a chisel, and then shoot. The length has been reached!
Now, not to gloat too much, but the shooting board really does "walk the talk". This board's edge that is in the foreground was at the exit side of the plane in the shooting board. I did not chamfer or relieve the edge in any way. There is no tearout.