I've never cooked a flat on a smoker in my life, so I'm sure there's others that can probably give you better advice. My only suggestion would be to get a few hours of smoke on it and then double wrap in foil with some broth.
If the flats aren't in cryovac, someone trimmed them. You might ask the butcher if he can sell them to you before they're trimmed or order them. You might have to buy a case, but if you have freezer space, they keep. I have nine in my freezer now.