I've seen many variations and wanted one where the pens/pencils were against the binding instead of on the cover. This design allows you to carry against the spine and open the sketchbook or notebook flat. There is a one and two slot option with three depths of grip depending on your preference. I keep a sharpie (13mm) and a mechanical pencil (11.5mm), so they're sized accordingly.
Designed for a Moleskine hard cover book at 2mm with tension clip.