The nice thing about the huge tank surface of the Rocket is pretty much any tank bag is going to work fine. I'm using a huge expanding magnetic Held. Running without a windshield meant I had to add straps to the front to keep it from lifting at speed.
I favour the Bagster/Baglux idea - fit a permanent tank cover and then clip on a tank bag to suit - anything from a map pocket to a huge bag. You can then keep the bags you buy and use them from bike to bike.