Well, the sugar babies I grew didn’t get that large, but I thought you could try to cross them with a smaller one.
I know you said watermelon, but I figured you might be doing something similar with cantaloupe.
Have you ever had a vine peach? They are tiny cantaloupes that are native to North America, very mild flavor, but could be canned in a syrup and likely would pick up whatever flavor syrup you used. They also might cross with something and produce an interesting product.
Vine peaches are on my list of “things to grow someday”. I hope to make a dent in that list this summer now that I have time and space to grow more.