Web Forms :: Create Multiple Masterpages And Create A Handler To Dynamically Set The Correct Master Page?

I'm currently in the process of creating a webshop. This webshop supports theming, but I also want customers to choose a lay-out. I can create multiple masterpages and create a handler to dynamically set the correct master page. But some components, like the shopping cart, are controls which have to be present in the master page and are also used in the aspx pages. The shopping cart control for instance, has an Update method which is called whenever the user adds a product to their cart. So the question is: can I somehow create a default masterpage which has all the components on it, but still be able to create multiple lay-outs. And how should I reference that master page from the aspx pages? I also thought of creating an Interface class which defines the masterpage and it's public components, but I don't know if I can reference an interface from aspx pages.