I cut multiple window mounts and mounted a mixture of A4 portraits and A5 which gives a good contrast between the images.

I mounted this A3 image by itself against a black window mount to give extra contrast to the light image. I made the window smaller to get rid of the white border o that the image directly juxtaposed the black card.

This is a A3 portrait image that I mounted this image and left the white border because I like the buffer between the dark image and the dark card.