RE: Roo speed brake ??
I stiffened my speed brake with a laminate of 1/8" balsa and 1/64th plywood epoxied to the inner surface. I have also seen a roo with what looked like 1/8" ply "ribs" glued perpendicular (90 degrees) to the inner surface. The ribs were only about 1/4" tall and ran to within about 3/4" of either end of the speed brake.