This display shelf is designed to be used with double sided adhesive tape.
The one picture shows two 320mm shelves side by side to make one 640mm shelf. the shelf pictured has a 14mm lip for attaching tape.
The slot is 4.2mm to be compatible with most of my line art uploads, which have a thickness of 4mm.
It is not necessary to put tape along the entire length, since the art pieces are very light. I just used a short 3cm piece of tape at either end of each section and that was more than sufficient!