Cut cucumber into 32 slices, each about 1/4 inch thick. Arrange on serving platter. For each canapé, place 1 piece of nori on each cucumber slice. Top each with about 1/4 teaspoon sauce. Scoop rice using 1 measuring teaspoon, pressing rice against side of bowl to pack into spoon and hold shape of spoon. Carefully remove rice from spoon; dip flat side into sesame seed and place, flat side down, on sauce. If necessary, carefully reshape rice with fingers. Top with bell pepper or other toppings (or a combination). Serve immediately, or cover and refrigerate up to 2 hours.