Hi Suzy. Yikes! I made this two or three years ago and recall that it is board mounted but don't recall making a notch where the palladium is. I happen to have the boarded treatment I made for this window before I made the one in the picture in my workroom. I just checked; the board is 2 1/2" wide and is not notched: I'm guessing that it's considered a 1 X 3. Again, I'm guessing that my installer use four L brackets, one at each end at the top of the trim and two more on each side closer to where the palladium starts. I could always go to my customer's house and look; she lives right in town.