Totally doable but not an easy job. Are you proficient in CAD (computer aided drafting)?? If so rough in a CAD drawing of the seat area of the bike. You will want to focus on the two areas of that seat frame that has threaded fasteners (the side grab handles and foot mount bracket install points).
Easiest way to measure things accurately is with a beam compass.
After you have the background in place you can design your own rack. I'd design for 3/8th tubing. Add lots of supports. Sheet metal is an other option but with those fixed screw mount orientations that won't be so easy.
Braze or Tig weld for the win.
Edit: This is sort of the type of thing I was thinking about. I will say, were it me, I'd prefer a bolt on that fit just over the top of that rear seat, and not cantilevered out the back. Remove the rack to haul two people. (Obviously that's a personal choice...)