Use clear sheet acrylic (Plexiglass) of 3/32" or 1/8" thickness, custom cut to the exact size of the posters. I go to TAP Plastics here in California to get this done. Clean the acrylic with Novus #1 cleaner after removing the protective covering. Don't even think about using glass or non-glare glass -- heavy, unwieldy, and potentially dangerous. Then get some 1/4" thick acid-free foam core (same size as the poster) for the backing. Order Nielsen metal frames from Westfall Framing in Florida

https://westfallframing.com/

Order a frame of the exact size (length and width) of the acrylic-poster-backing "sandwich." Don't add "extra" onto the length and width; Westfall does that automatically. Make sure the particular frame style you order will accommodate the thickness of the "sandwich." Do not order a frame style that has a narrow face (like style N33) for such large posters because the sandwich may not be held firmly enough and it will tend to fall out of the frame. Westfall also sells foamcore backing (that you can easily cut with a razor blade) if you can't find it locally. The Nielsen frames go together easily and perfectly with a screwdriver. The frames come with hangers that go on the back sides of the frames. Use vinyl coated wire from Amazon of the correct strength to hold the finished product

https://www.amazon.com/gp/product/B00HZHV582/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

and a wall hook or a cleat nailed into the wall. I have framed hundreds of photographs, prints, and posters for our house and for other people, ranging in size up to 6 1/2 ft x 3 1/2 ft using this process and have never had a failure. Westfall is very responsive and you can even reach them by phone if you have questions. Great posters! Good luck!