Real easy, build in glass, add ice, top soda water, quick roll in small tin and done.
I'll add the recipe for the Watermelon syrup in the notes.
Watermelon Syrup:
Half of a basketball-sized watermelon (I'll get you some weight measurements soon) cut into inch cubes and blended to a pulp with one jalapeño and fifteen mint leaves.
Fine strain and add one barspoon or roughly one teaspoon of coarse sea salt. Add one pint white granular sugar. Whisk briskly with whisk until combined.
Recipe should produce two quarts of syrup.
Flavor notes to expect and tweak at your leisure:
It's a bit thin and the sugar plays strangely with the low natural sugars in the fruit. Texture is silky foamy beer. Honestly because of added sugar making watermelon flavor feel "pushed," I wouldn't add more sugar to make a 1:1 syrup. Does well in cocktail by playing up agave in both added syrup and tequila. Holds body in soda.
The Gadsden Purchase is a 29,670-square-mile (76,800 km2) region of present-day southern Arizona and southwestern New Mexico that the United States purchased via a treaty signed on December 30, 1853, by James Gadsden, U.S. ambassador to Mexico at that time.
My current Chef is from the area and the name stood out to me through Wiki blackholes.