I made a box clamp out of 1" x 1.5" Spruce I ripped from a 2" x 4" on the tablesaw.
The back piece is the length of the box. The 2 sides are 0.5" shorter than the sides of the box, plus 1" for the width of the back piece. The front piece is 4" longer than the box.
I pre-drilled all the pieces, and screwed the back and sides down to the bench with 3" screws, squaring as I went.
I also pre-drilled the front piece and used 3" screws to screw the front to the sides. As they are 0.5" shorter than the sides of the box, tightening the screws clamps the box together.
I also staple-gunned wax paper from an old cereal bag inside the box clamp so the glue doesn't stick to it!
It worked flawlessly, love how it doesn't move around...
Think I'll leave it screwed to the bench for when I glue the soundboard and back on.