To make the boxes I use a screw clamp and a right angle clamp.
The right angle clamp ensures the corner to be right angle. A pretty useful tool.
With this method I build up to 120 boxes per day. Sufficient rock music, coffee and chocolate provided.
I use the Gatineau wood joints as shown in the pictures below. He uses a rebate all around three sides: left, right and upper (as a frame rest). The rebate has the size of the wall thickness.
This sort of joint produces a strong connection between the walls. With Warré boxes with nailed topbars the use of those rebates are not necessary. Because the nailed topbars stabilizes the box. But the joint Gatineau uses has another advantage: it hides the facewood which provides protection against rotting and weather.
A short video on how I assemble a box: