Blank slide mailers for holding 2 microscope slides. They're meant for safely shipping microscope slides, but the little cut-outs make lovely little archways or windows. Note the little notch cut from the top right of the cover (when closed).
Many collage sheets have been designed with images to fit these mailers.
Sold individually (just the chipboard mailer... no glass included).
Sample artwork is by Sarah Whitmire also using images from the Animals in School Collage Sheet.