Create a "Add to Google Calendar" button for iCal feed

I run a website where we make listings for upcoming shows available in a variety of different ways. We are currently developing an applet that allows a user to create a custom feed. For example, if I would like to subscribe to a listing of shows in my neighborhood, I would use this button to create that feed for me. I can subscribe to it as an RSS feed, or as an iCalendar.

We created the iCalendar option because there was demand for it from some of our users. However, most of our users are not going to understand how to subscribe to an iCalendar. I would like to create a "Add to Google Calendar" button that makes this process easy for anyone who uses Google Calendar. We have too many possible combinations of options for us to go to Google and make these calendars publicly available.