half lap is the way to go
I setup my dado blade for width of the spacers, clamp them together on the sliding table and cut all x then y at once - spacing does not have to be dead on - you just need the right number of slots for the number of right angle pieces - make the length < 1/4" or so short - they do not have to butt up to the outside edge pieces
get the 2 top & bottom panels sized, cut your outside ribs accurately - here is how I assemble it with no musss nor fuss
I use a pin nailer to assemble the outside ribs in a square over one of the panels - just nail them to each other - put your internal ribs in with your half-laps - just run a big old bead of yellow glue on the top of each rib - put the other panel on top and pin each corner - have a helper assist you in flipping it over - take the other panel off - run a bead a glue on the rib tops - pin the other panel back - pop into a vacuum bag and let it rip for 30 min
you are done
Torsion Box stretcher direction.
The direction of the solid pieces usually run the long direction which is opposite from your sketch. I dont know if this is an error or modified for your particular use.
When I have made large torsion boxes in the past, I dado'd the bottom sheet of MDF to help line up the long strips. The spacers were cut shorter in height so that all of the tops lined up. By doing this I could line up my pieces with glue under them, set them in place and and then add the top without anything sliding. Then brad nail the top and slowly slide the whole thing off my workbench brad nailing the underside as I went along. This allowed me to make the entire torsion box top by myself all in one sitting.