Thanks!
I'm too cheap to pay for the specific plate. I used a scrap 3/16" aluminum sheet that I had and mounted it where the luggage rack used to mount (about 10" x 10" depending on how you do it). Then mounted their generic locking plate to that sheet. That stays on when the trunk is removed and looks pretty good.
I think the 3/16" is thick enough, but I might have gone with 1/4" if I bought a piece. I get a little bit of flex because the trunk is mounted almost completely "behind" the rack mounts. That is so I can leave the backrest on. If you remove the backrest, you can mount the trunk further up and will be more stable.