Funny you mention that!
My new to me TT was missing the panel when I got it.
Amongst my tools and equipment on my truck is a portable aluminum brake. And a roll of coil. and the tools needed for the task.
I took a few critical measurements and laid out the piece of metal, cut it, folded a few hems here and there, a little measuring, a bit of trimming to suit and voila! On really nifty sheet aluminum sliding panel that works far better than the plastic factory one.
I had to partly disassemble the screen door to get the thing in there, but it works like a charm!
Now here is the kicker - I had just bought the TT and was on it's maiden voyage. I was going through the drawers of misc. stuff left behind from the previous owner and found this curious looking square black thing in one of the drawers. I scratched my head, wondering what it was and put it back in the drawer.....
The next day when I was marveling over my handiwork, I suddenly realized what the little black plastic thing was!! After all that work making a new one.
But I came to realize why it resided in a drawer instead of on the screen. It was too small and fell out.
Gee, maybe I should mass produce them and give them away to folks like you!!
Sorry OP, I realize this does not answer your question, but thought you would find it interesting.... maybe.... :B