I spent about a month trying to come up with an easy way to build these torsion boxes. The half lap dados are tempting, but just try to get them all in 3 dimension parallel (alignment) before the glue dries. I tried twice, and gave up. That is why is used the individual spreaders. Gerry’s boxes were a lot larger than mine and he used screws to hold them together. If you are trying to build thinner boxes, I found that the parts floated around a lot.
I used the base ply or MDF as the ‘base’. Double check this for square. If it was ¼ inch ply, I used double sticky tape to make it lay flat. Then I glued the first stretcher and checked for perpendicular while using my finger to make sure that it touched the 3 edges of the base. I then spread glue on the first set of spacers and squished them into place (placement is not important). I then spread glue on the second stretcher and squished it into place, again checking for perpendicular and using my finger to make sure that it touched the 2 edges of the base. Repeat this along the entire base. The last set of spacers never came out to the right length, so I custom cut them at this point to make sure that the last stretcher was just touching the far edges of the base. I then let this dry overnight before adding the second skin by applying glue and a weight to hold it in place until it dried.
I did not use a nail gun or other fasteners. Only glue holds the box together.
Just my $.02