Strong and stiff is always good but watch for weight buildup. At this size and for optimum aerobatic performance weight is truly the enemy. Look at the weights of the flat foamies when in ready to fly condition and figure that you're going to want to keep things to around that sort of weight. Also if using foam in a manner other than in a flat plate it is actually darned heavy. If you're adamant about a built up shape and must use foam then do consider using a folded over construction of 3mm Depron with a few 3mm Depron ribs and a 3 mm spar that is eggcrate notched to the ribs. A small diameter carbon rod (like .050) can then be sandwiched between the full depth spar and the outer skins to work as the spar system.

For diagonal strengthening some internal or external diagonals of carbon tow strands would provide a lot of stiffness. With this makeup all you need to bond the two fibers would be some water based polyurethane varnish. Also with the depron skins and internals there's no need for the heavy glass cloth and resin outer coating.

The other option would be a full on vacuum bagged wing with the glass/epoxy outer skin. But in that case to avoid an excess weight buildup from all the foam in a thicker section I'd suggest going with an 8'ish % airfoil to cut down on the foam volume.

Or.... the old curmudgeon that I am... there's always the old tried and true balsawood option. Ribs with balsa leading and trailing edges, spruce or balsa/carbon laminated spars and then work in a full geodetic XXXX format of carbon tow diagonals. Finally tissue and dope or use one of the light parkflyer films. Do the same sort of built up structure for the control surfaces as well. This would be both light and stiff but it would look "old fashioned" but what the heck. It'll work and work well while being light.