Based on your discovery of the apparent unavailability of a pillion mounted luggage plate, the easiest option seems to be to just make one. I've made a couple luggage racks for myself out of 3/16 thick aluminum, which is available from a vast selection of online metal suppliers (including eBay). If you're up to the task of making your own brackets, then making the luggage rack itself shouldn't be an issue. It doesn't have to be aluminum; HDPE (the stuff they make cutting boards from) is a good choice as long as you get a thick enough piece. Guys on here have made luggage racks out of actual cutting boards.
This is a luggage rack I made for my old V-Strom; it was the first one I ever made.
This is the current one that's on my Super Tenere:
They're both just flate pieces of 3/16" aluminum sheet. They were cut out using a hacksaw, a handheld scroll saw, and a coping saw with a coarse blade. There aren't any bends to make; the angled sides are just hardware store aluminum angle, bolted to the sheet. Right now there's a 16" by 3 foot sheet of 5/32" aluminum for sale on eBay for twenty bucks that's big enough for multiple luggage plates.
A pillion luggage plate would be even easier to make. I doesn't even need a complex shape; a rectangular piece would work fine. You can make slots around the perimeter for attachment points by just drilling two holes for each slot and joining them with a scroll saw.
Seeing a piece of aluminum with holes cut in it that cost almost $200 is what caused me to make my first luggage plate.