Preheat the grill. Combine the zucchini, squash, onion, tomatoes and cheese in a large bowl. Toss with 2 tablespoons of the olive oil and season with salt and pepper. Take a piece of aluminum foil that is at least 24 inches long and brush with the remaining olive oil. Place the vegetables and cheese on the aluminum foil. Fold up the aluminum foil so the contents are sealed in. This can be done up to 4 hours in advance and stored in the refrigerator. Place the packet on the grill. Grill for 7 minutes. Flip the packet over and grill for another 5 to 7 minutes. Remove from the grill and let rest for 5 minutes before opening the packet. Open the packet and sprinkle with basil.