I don't have a recipe but if I was going to start work on one I'd start with TPA 'Cotton Candy' (which is basically just 10% Ethyl Maltol I believe) and Cap 'Sweet Watermelon'. I much prefer TPA 'Watermelon' but I'm not sure it would work well for a cotton candy flavour.
Maybe start with some basic cotton candy recipes (there are tons of strawberry ones about) and replace strawb with different watermelon flavours to see what works best.
From what I've seen, most cotton candy recipes are relatively simple - eg. EM and one or two other flavours.
Hope that helps in some way and I hope you find that awesome vape you're looking for!