Hello!
Do you not just have a smallish "standard" dry bag that you could mount "sideways"... like 99% of people from 1890 to 1995 did...? Couldn't be cheaper or lighter and no need for a frame/harness to stop it "wagging" like soooo many were at this years WRT
This works side-ways-on for my younger one just fine as there was no way you could fit one in the modern "length ways" without fouling the rear tyre/wheel.
You need 1 x dry bag and 3 x straps. Two straps round the bag and under each seat rail. Then you need a slightly longer strap that you loop round the seatpost (bottom), pull backwards (so middle of strap is around seatpost and you have two equal length in your hands),pull under the dry bag, over the top of the dry bag, loop round the seatpost (top) and back round on its self to tighten. This holds the dry bag from rocking back and forth. Think of the long strap ac a kind of "C" shape when looking sideways at the bike....
Could save you forking out on something you just don't need?
Here is a picture off the internets to show what I am getting at. The bungy cord in this picture is being used instead of the 3rd longer strap.
