Well, that's going to be beautiful! Adding 1/2" won't work, however, you need to base everything off the seam line on both pieces. Figure out your seam line on the fan (so 1/4" in) draw the seam on the background then add a 1/4" to that. When you are done, the curves will look totally different on the 2 pieces, but they should seam up OK. Here's a link to a drunkard's path template. You can see that the curves are different:
http://www.everythingquilts.com/brow...Templates.html
You will need to starch the background so that big curve doesn't stretch, and pin like you are easing a sleeve in. I would do a trial run with a couple pieces of scrap fabric. Another alternative is to applique the fan onto the background.