I have finished making my top tube bag, frame bag, B17 bag (under the leather but above the rails), and rear triangle bag. I have heaps of material left over, so I was thinking instead of buying a seat bag like the Pika or Viscacha. I can use the left over material and make a dry bag harness. Currently I have a 20L dry bag, although I only need 12L of storage so I will roll down the dry bag a fair bit, and it weights 2.3kg (5lbs) when filled with my gear. It will have to be attached to a carbon seat post, with a B17 saddle, with 8in of saddle rails to wheel clearance.

I don't really know how to go about making this so any help or advice would be great, and patterns or plans would also be quite helpful. I want to try to maximise the space at the seat post end of the bag/harness, I don't want to make a really long one, as I am concerned about levering force on my carbon seat post.

Also is 2.3kg (5lbs) to much for a saddle bag?