I did a prototype for a ripoff of the Wayward Lousie harness while back, used an Ikea cutting mat. A tad on the small side but seemed promising, other stuff got in the way, might pick it up again this summer. I can send you the Illustrator file / pdf / svg if you want?
Seat post mounted bike packing bags - I saw a link once to a template to make a holder that the bag goes in. Cant find it anywhere...
Any help?