An extremely simple pen tray that mounts to the bottom of a computer monitor. Affix using standard adhesive strips.
The model is 15cm wide, but can easily be scaled to any size needed. Would work equally well attached to any other straight surface such as a shelf, desk, whiteboard, etc.
Seriously, this might be the most simple object I've ever posted. No supports, no post-processing. Just print it at whatever settings you like, put some adhesive strips on it, and stick it to your monitor.