1. Using a large chef’s knife, cut the watermelon into two equal halves, lengthwise.
2. Use your knife to slice all the way around the perimeter of the watermelon flesh, about an inch from the rind. Be careful not to pierce through the skin, as this will lead to your Jell-O leaking out. Then use a kitchen spoon to scrape out the rest and create a smooth bowl with roughly 1/2-inch red watermelon flesh left on the rind.
3. Line two small, rimmed baking sheets with paper towels in a way that creates a bed for your watermelon bowls. The idea is to create enough support to keep the watermelon bowls from rolling at all while they’re in the fridge. Put one bowl in the center of each tray and set aside.
4. In a large mixing bowl, combine the Jell-O mix and the unflavored gelatin. Add 4 cups of boiling water and whisk until the gelatin is completely dissolved, about 2 minutes. Add the vodka all at once and whisk just to combine.
5. Pour the Jell-O mixture into the watermelon bowls, dividing evenly. Carefully place the Jell-O filled watermelon bowls in the fridge, and refrigerate until set, about 4 hours.
6. When Jell-O is set, cut each half into 1-inch slices, crosswise, then cut each slice into 2–3 wedges, and serve.