Because it's small, a Cavaillon melon makes a perfect bowl for a single serving. Here it's filled with scarlet raspberries and watermelon, and the orange Cavaillon melon itself.
Cut off top quarter of each melon using a round chisel or sharp paring knife to cut a scalloped edge; scoop out seeds. Working over a bowl to collect juice and using a 7/8-inch melon baller, make 2 cups of melon balls from top and bottom of melon; add melon balls to bowl with juice.
Working over the bowl to collect juice, make 2 cups melon balls from the watermelon, avoiding seeds. Place watermelon balls in the bowl, and remove any seeds.
Add raspberries, port, sugar, and lemon juice to bowl, and toss well to combine. Let stand about 15 minutes.
Spoon melon mixture and juices into carved melons, and serve immediately.