I think there are a few options for a tank bag but I'm not sure if any are designed specifically for the Sled.
I'm using a Wolfman Explorer Lite tank bag that I've had for a while and has been on several bikes now. I purchased another mounting harness from Wolfman (Universal model harness works) and leave it on this bike as I swap the bag between the Sled and my Husky 650. It fits pretty well and I like the size of this bag which does expand quite a bit. The bag buckles to the harness at 4 points so it's easy to unbuckle the front and tilt it back for refueling or to undue all 4 buckles and lift it off the bike.
Here's a shot of it on the Sled not expanded.
Here's a closer view of this bag on my old KLR650;