According to the headline, I am looking for a large board as a passepartout, or how I can solve the problem in another way.
I am a bit unsure of the measurements, but it's an old, battered school chart measuring about 137x157 cm. Ideally, I would glue it onto some simple "board" to reduce the weight of the whole thing. Since one (I) doesn't find boards wider than about 120 cm in hardware stores, I'm asking here how I can solve the problem. Is there a specialty store that might have boards in these dimensions (it's not extremely large), or should I figure out a good lightweight construction with a frame and two adjacent boards to make it work? I can provide the exact dimensions of the poster later, but it's too wide for a regular 120-width, and I would like to see about 3-5 cm of the passepartout, which in other words means the board should be about 6-10 cm wider and longer than the school chart.