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I did think about just using the chainstay mounts but it would have meant a very different angle for the dry bag or an extra bend in the tubing plus this way I can also swap the whole system over to another bike like my hardtail which doesn't have mounts. As for the frame bags, I don't think zips are inherently a worse option as long as it's a good chunky zip and not one of the waterproof ones. For example the water-resistant one on my half frame bag is pretty much knackered now after quite extensive use despite me cleaning and lubing it. On the up side I now how to replace a zipper so the bag itself should still live on. The roll top bags also take a bit more work to stop the contents bulging outwards and interfering with cranks etc. when you don't pack them carefully.
Big fan of all of this! But I’m wondering why you went for a second seat post clamp with the Porcelain Rocket knockoff. The bike has rack mounts on the chainstays, no?
And what is your rationale for rolltop framebags over zippered versions?