The radopen method does exactly what you need and also the best place for the RadWindowManager is the MasterPage as you have done. What you should do now is to set the desired common properties to the RadWindowManager and then open RadWindows with radopen(url, null) - this will create a new RadWindow dynamically on the client. If you want also to destroy the created client object for the RadWindow when it is closed you can also add DestroyOnClose="true" in the RadWindowManager declaration.
All the best,
the Telerik team