Buggy,
I like Mid's suggestion if you plan to put two of them on. If you only to put one on in the rear, I'd suggest looking at placing it underneath one of the rear lights. I think the size of the logo might be a bit squeezed (will looked wedged in) if you try to put it next to the plate in the back.

I'm waxing the XF today, so if you send me dimensions, I'll play around with it a bit and let you know what I think.

Nice idea.