Great images, Desk.
I think your third pic, that is closest to your "test page" shows this would work OK with your framed posters. it looks like sample you have has a slight texture, almost, to it. But it looks to be crystal clear when close to the test sheet. If the price is right, I would consider going with something like this.
And plexi, with even some UV protection, is the way to go, as even indirect lighting (or indirect sunlight) can speed up fading, over time.