Spread 1 tablespoon marinara sauce on each rectangle to within 1/2 inch of edges. Place 8 slices of pepperoni in center of each rectangle; top with 1/3 cup cheese and 8 more slices pepperoni. Bring ends up over filling, overlapping about 1 inch. Press edges with fork to seal. Brush each sandwich with water, and sprinkle with Italian seasoning.