I say hack it!
Undertails look great, but they are WAY too expensive. And then the license plate covers most of the good looks anyway, so I suggest saving the money and hacking the fender.
I have also seen some people take their whole tails out (battery box and all) and paint the bottom to match their bikes. I find it looks better than normal, although not nearly as neat as an undertail. I have also seen the bottom covered with thin sheet metal that is painted to match the body. I think this looks great, and it still inexpensive. You can just rivet the sheet metal to the tray and it looks good.
I have also seen some people take their whole tails out (battery box and all) and paint the bottom to match their bikes. I find it looks better than normal, although not nearly as neat as an undertail. I have also seen the bottom covered with thin sheet metal that is painted to match the body. I think this looks great, and it still inexpensive. You can just rivet the sheet metal to the tray and it looks good.