Adding Protective Inserts to Window Boxes?

I always enjoy Ellen's gardening contributions to the Thriftyfun column and just subscribed to the newsletter. Is there an advantage or disadvantage to having the window boxes built to hold a purchased porch/window box as an insert? I've always wanted to have one window box on a double window on the front of our home and visualized having a open or screen type bottom so that I could use a removable box inside that would protect the wood of the real window box. I hope this isn't too involved a question, but this seemed to be a good place to ask.

We have lots of that trailing vinca vine in our gardens that would look so nice trailing down from a window box. It was interesting that you mentioned that the box should be as wide as the shutters. I wouldn't have thought of that. Thank you.

By Mary A.