Mockup Backer -

The industry standard. It is lightweight, easy to cut with an X-Acto knife, and available in various thicknesses (usually 3/16").

Allows you to use various adhesives, from spray glue to archival mounting tissue. How to Apply Your Design to a Backer mockup backer

Thicker backers like 1/2" foam core create a "drop shadow" effect against walls, making the art pop. The industry standard