How can we create custome dialoge boxes?

We are developing several widgets and would like to display a cusom dialogbox  with cusom buttons centered on the screen. Currently we are using modal form, but it is very small and limited to the widget area. Providing a full-screen or centrally positioned custom dialog box would greatly  improve usability and help the users to interact with DNG in more automated and intuitive. How can we create popup custom dualog boxes?