Ephemera can broaden your collection in more ways that one, eh?
Two sheets of 1/8" thick clear acrylic plastic don't cost much; here in the US, Home Depot happily cuts them to size ... which should be, say, four inches larger, in height & width, than the poster.
You will need to give the poster something to rest on, so's it does not fall out the bottom ... cloth ribbon of the sort with which women once secured their hems will do nicely, and comes in a variety of colors, and you can double it up to thicken it.
Place the ribbon across what will be the rear-most sheet, about two inches from the bottom. ... you can use a simple water-vulnerable household glue like Elmer's to fix the ribbon in place and, if need be, easily remove & replace it.
Lastly, place a poster in between the sheets, and use UHU's removable gummies (below) in each corner to mate them for less-than-life.
I'll leave it to y'all to work-out how to best hang/rest that arrangement, and how often to swap-out one poster for another, and what color ribbon work best ... white would disappear nicely, but a complimentary color shows good taste.
/s/ ikeyPikey (who has been mulling what to do with the magnificent & ridiculous over-size covers he bought out of a bargain box at NAPEX)https://www.uhu.com/en/product-page...-yellow/4948https://www.uhu.com/en/product-page...visible/4949https://www.uhu.com/en/product-page...ix-deco/4169http://elmers.com/product/detail/school-glue