From The Art of Drink (https://www.artofdrink.com/ingredient/cream-of-coconut-recipe). We mistakenly used Coconut Cream instead of Coconut Milk (so higher fat) and we found it a bit sweet, so might knock down the sugar next time. But it's interestingly almost as easy as homemade Rose's Lime (which is itself basically 1:1 simple syrup with lime juice instead of water).
400 ml can of Coconut Milk
375 g granulated sugar
Potato starch slurry (make about 1 or 2 oz)
1/4 tsp salt (to taste)
Warm coconut milk, add sugar, and stir until sugar is dissolved. Add salt, to taste. Bring to low boil, turn off heat. Add a couple of teaspoons of the potato starch slurry and stir. Let the mixture sit to thicken.