A simple holder for round canvas panels. 

Usage

1. Open the FreeCad file and set the parameters in the Spreadsheet according to your canvas dimensions (diameter, thickness).
2. Export to stl and print two pieces. 
3. Place the pieces facing each other on the edge of the canvas and tightly tie a string between them (like in one of the photos).
4. Tie a loop with which the canvas can be hanged on the wall.

I uploaded an stl for a canvas with 150mm diameter and 4mm thickness.