Using a cake stand flipped to its top, adjust your watermelon on the cake stand base where it will be sturdy and balanced. Using a small knife mark where your spigot will be ensuring not to cut fully in to the watermelon. Also mark where your top will be cut to ensure a straight cut.
Remove the watermelon from the cake stand. Cut top off of the watermelon. Scoop out flesh leaving a thin layer of red.
Cube and freeze half of the watermelon flesh. Puree the remaining watermelon flesh and strain the pulp and seeds using a mesh strainer. Keep the strained watermelon juice.
Using an apple corer, remove the portion of the watermelon where your spigot will be inserted. If the rubber gasket will not fit, thin the wall of the watermelon a little bit at a time until it fits. Avoid making it too thin as the wall of the watermelon may collapse. Refrigerate empty watermelon upside down in colander until needed.
In a large pitcher, combine tequila, triple sec, margarita mix, and watermelon juice. Refrigerate until needed.
When ready to serve, ensure the spigot is in the off position and slowly pour margarita mix into watermelon keg. Add frozen watermelon cubes.
Garnish options: salt rim glass, jalapeño slice, and or fresh lime wedge.