Did you try to copy a current design (i.e wildcat tiger’) or just wing it?
I took the pattern for mine from the link below, but my translation was not thought through properly.
My current set up has no harness just 4 webbing straps. 🙂
http://www.bikepacking.net/forum/index.php/topic,2611.0.html
TBH for the work involved in thinking it all through and maybe making a prototype or two I would say it’s worth in this instance just buying one.
Another consideration is, unless your machine will handle a thick thread is that the harness unlike the framebag has a good deal more stress on the stitching. I exploded my harness stitching quite easily cinching it all down nice and tight even though it had all been double stitched.