Assuming you have some metal-working, experience just fabricate it.
.100" aluminum angle and pop rivets. Very easy to work and very strong and not too heavy.
The sheet came from somewhere else. It's scrap from something. Might get it on ebay.
Angle and thin gauge sheets can usually be had at Home Depot or Menards.
We slapped this together in a couple hours and it was super strong. I wish I had more photos during construction, but it was done quickly. All we used was a jig saw, drill, maybe a few files and a pop rivet gun.
If you've never cut, ground, drilled fitted metal parts before, then maybe this should not be your very first project.
The Kz650 duck tail is the most iconic, in my opinion, of all of the Kz tails. It is the most artistic so it would be a shame to mess it up. Of the photos in your other thread about this subject, your bike looks way better than the black one you posted.