In my experience, any kind of glueing w,ithout wrapping around the corners does not work very well on flexible substrates (and in my understanding the black plastic cover is flexible, no? )
Sooner or later the corners get loose/dirty/peel off...
I would cut a window into the black plastic cover and put an interesting/surprising motif onto the first inner page, of which the window only shows a detail.
Looks neat and clean + surprise effect.