I've been looking for a frame bag for my Horsethief since I picked it up. I was some what disappointed that Salsa didn't already make one. A custom bag was in my future but I made a discovery today, we had a Surly fat bike frame bag floating around the shop and it looked like it might fit. A couple of straps removed and a few trimmed to fit and I had a great fitting bag. As a bonus, the fat bike bags are a bit wider especially at the head tube, so it helps make up, for the smaller size of the bag. For reference my Horsethief is an extra large and the bag is the Surly Frame Bag 1.

Before trimming the straps.

All straps trimmed and neat.

Happy trails...