If you do not already have a custom fit frame bag for you Mukluk then you better get to local Salsa dealer for one now! These bags are made for Salsa by Revelate Designs out of Alaska. Revelate Designs also has pre-made bags for Surly and Fatback available through retailers of those brands. Revelate Designs fabricates other waterproof storage accessories for you bicycle such as the Gas Tank for your top tube, the Viscacha seat bag that expands from 6-14L, the Harnes to carry large dry bags on you bars, Tangle bags (smaller frame bags), and custom frame bags made to order.
I am the proud owner of a few Revelate Designs bags and will be writing up a review shortly.