There are a great variety of ways to attach art into a window mat. Corners are a common method, they are usually made from paper or polyester strips.
Paper corners are an easy and reversible option for securing artwork into a mat without applying adhesive to the object. A strip of preservation quality paper, compatible with the artwork is simply folded into a right angle (or more appropriate angle if the artwork is not straight), and secured underneath with small bits of double sided tape, or over the corner with an approved linen or other tape. The paper can be folded around matboard scraps for items with more dimension. Although the corners will cover a bit of the artwork by necessity, you can use translucent papers, or cut part of the front corner into a square shape to allow the art to show.
Fold-out top corners can allow access to the object’s verso while still providing a secure attachment under a window mat. Attach only the lower leg of the corner, and leave a long tail on the upper tail for each top corner. Label any corners that are designed to fold out.
Sometimes for thick items in a sink mat (especially if they have uneven shapes), strips of polypropylene will be used to make corners or corner straps around the item. These can be passed through slits in the backboard and attached on the verso. Some polypropylene corners are available commercially, but these should be carefully evaluated for adhesive stability. Often the adhesive comes to the very edge of the corner, and adhesive may creep onto the verso of the artwork if measures are not taken to protect it.