Add enough watermelon to a blender to come almost all the way to the top and add the next 2 ingredients. Blend until smooth, about 20-30 seconds. Add the a few mint leaves and 2 slices of lime to a glass and use a wooden spoon to "muddle" or bash it up. Add ice and pour in the aqua fresca.
Repeat this process 4 times for a group of large people.
To make the keg, slice a tiny piece off the bottom so the watermelon will stand up straight, too deep and the aqua fresca will leak out.
Slice a big enough piece off the top so you can scoop out all the melon using a large spoon.
At the location you want to insert the spigot, scrape as much of the inside part of the melon away as possible. Use an apple corer to create a hole for the spigot. Make sure you place the spigot in firmly and crank it as tightly as possible.
Pour the aqua fresca in the melon and use the top as a cover if you desire.