In case it helps anyone, the Tailfin 3L Half Frame Bag is a perfect fit for the Cannondale Topstone Carbon size 56.
Also works well with the top tube bag. When this is bolted on, the position still allows the strap mounting points to be used and shared with the frame bag (meaning you can easily remove the frame bag and leave the TT bag in place).
I spent a good bit of time messing about with paper templates before ordering but still wasn't quite sure how it would work out.
by MonsterMunch00