The best make-your-own version is to mix 1 cup of evaporated milk with 1-1/4 cups of sugar in a saucepan, heat and stir until the sugar is completely dissolved, and let cool. If you don’t have evaporated milk on hand either, you can make your own by slowly simmering any quantity of milk in a pan until it reduced by 60%, and then adding the sugar.
Obviously this is a tedious process. Sweetened condensed milk has a shelf life of about a thousand years*, though, so pick up one or two cans on your next trip to the market and you’ll never need a substitute again (it’s a good idea to turn the can over every two months or so, to keep the sugar well dispersed throughout the milk).
*OK, it’s more like two years, but it’s still a fairly long time….