Yea, hinged flexible plexi is what I'm thinking. Down the road I will have it painted and fixed up. I dont have the $$ for that just yet at least the paint unless I just rattle can it. But we'll seePersonally, I would sandblast that tailgate, weld up the cracks and paint to match the truck.
For plexiglass back window, are you talking about temporary? You could try something like what @76zimmer did, but with a hinged or flexible piece at the bottom (out of clear, not white of course)