I like the idea and the photos.
I do it a little differently on my box jointed boxes. I build the sides, ends and back. Make sure the back is dead ass square and sized perfectly. On my builds, the back is inside the sides and ends. Then I dry assemble everything, including clamps. I use one of those webbing clamps around the perimeter and then 4 clamps across the sides/ends.Be sure the assembled box is on a very flat surface. Dry assembly including clamps is imperative. Nothing worse than trying to figure out how the clamps will go as your glue is rapidly drying. So, in my method, the back acts as the method of making everything square.
The glueing procedure is not as simple as one would think.
PS It is nice to see some activity on this group.
Your home-made box clamping jig looks pretty good. The one short-coming I see is that when you tighten up the front board, it tightens the sound box frame you are gluing up only on the two long sides of the box, but not on the two short sides of the box.
I was thinking that if the clamping jig were designed as two "L" shaped pieces that clamp together to make your rectangular form, then pressure would be applied on all 4-sides. Both "L" shaped pieces might be 1/4" short on both sides so that a 4 sides receive the pressure. The underlying board that the two "L" shaped pieces are screwed down to might have to be cut diagonally with a 1/2" gap so there is space to allow the two "L" shapes to be clamped together under pressure, but still retain a perfect 90 degree angle on all 4 corners. At this point its a "brain exercise", but maybe I can build one to show you better what I mean, or at least do a drawing. Let me do the drawing first.
T- nuts easy to find at the local hardware store. Epoxy them in or they will loosen up and fall out. Very handy items.
Here's the drawing I promised... In my drawing I was assuming we'd be using 1" x 1" stock for the jig. The two triangular base boards can be made from 3/8" plywood of suitable size and cut on a diagonal as shown in blue on the above drawing. I showed wood screws, but the fasteners you guys have been discussing are probably better options.
Remember, if all sides are cut to exact length, you only need to be conerned with one right angle. The others will fall into place automatically. Basic geometry.
that is supposed to say concerned....
I use my old shop mate n' a couple bar clamps, but I need six hands.