After much messing about making things overly complicated, I figured out a simple useable design that works.
I just need to work on a better way to secure the bag to the post in order to stop the slight bit of moving around
that still happens. My goal is to have it SECURE so there is no movement in the roughest trailriding conditions.
Like my Handlebar harness this predominantly made out of stuff I had lying around at home and work.
The harness body is from the same 1/16" pvc sheet I found at work. I also made a bracket out of a rectangle of
1/8" aluminum from somewhere
A hole was drilled in this aluminum and after some trimming and bending, the bracket was placed under the seat across the seat rails.
A hole in the pvc (I put a banner grommet in the hole because I can) and large washer on the underside of the pvc harness, along with the right sized nut and bolt (bike parts) holds the whole thing tight and stable.
I use a 2" wide velcro strap bought from Home depot. It goes through the rails and around the harness. This takes
the hanging/flopping forces away the plastic nut bolt area as the seat rails and strap. As it is, it is actually more stable than the store bought way smaller bike bag attached to my road bike.
I Just have to work out the post/bag connection and I'm off to my test track for some R&D pummeling:)