The Tailwind has no diagonal bracing, the plywood sheathing carries the torsional load. Same with the Pik 26 wing in the two photos I posted.No diagonal/drag strength. Take every other rib and add some angle inside of the spar box to prevent the structure from paralleling.
You can use paper over the solid foam but it dents easy. Works fine tho and paints with latex well.
In the case of the sketch none is needed as the solid foam caries the torsional load. Peter used no diagonal bracing on his and it was not even solid foam.