Last time I fitted some FCR to a 900SS I took some photos of the airbox trimming for future reference and thought I might as well post them here too. To do the trimming I use an electric die grinder with a very coarse conical bit. It looks rather medieval, but when it's honking around at 26,000 rpm it's amazingly easy to use gently.
I find you can refit the breather hose to the airbox too, even though the Sudco instructions say you can't. It can be routed under the throttle cables. You need to round the edge of the box below and behind the breather hole to get it all to fit nicely, but if you go gently you can make enough room.