Preheat oven to 180°C/350°F. Add 2 tbsp of coconut milk to a small saucepan and set over medium-high heat. Bring to a vigorous simmer and cook, stirring occasionally, until the fat separates from the liquid - about 2-3 minutes.
Add the curry paste and cook until very fragrant - another 2-3 minutes. Add the remaining coconut milk, brown sugar, fish sauce and lime leaves. Bring to a simmer and allow to simmer for about 5 minutes.
Meanwhile, cut 2 30cm (12in) pieces of aluminium foil and lay them on a clean work surface. Mound the rice equally in the centre of each piece of foil. Arrange the sea bass fillets over the rice.
Remove the red curry sauce from the heat and stir in the Thai basil. Spoon the sauce evenly over each fish fillet and crimp tightly to completely seal the fish and rice in the foil. Place the packets on a rimmed baking sheet.
Move the fish to the oven and cook until the internal temperature of the fish registers at 55°C/130°F when an instant-read thermometer is inserted into the thickest part of the fillet - about 15 minutes. Allow the fish to rest in the packets for 5 minutes before un-crimping and serving, topping with more Thai basil if desired.