Ecommerce module customization

Hi, I would like to add some custom methods to the Ecommerce's Product model and I don't know which will be the best way to do it. Can I ask you to provide me a possible solution? I know that if I add a method into the Corals/modules/Ecommerce/Models/Product.php then I will lose every modification after the first update. I thought that I have to create a custom modul where I will extend the Product model and I will add my own method here. Thank you, Szabolcs