Inspired by Instructable:
http://www.instructables.com/id/Clamping-Squares/

Use this tool with standard clamps to maintain in position a picture frame. Second picture of the gallery shows the installation principle.

If you want to print this tool, you can use the Customizer to generate the stl file sized according to your needs.

However, I recommend cut the tool in wood, using laser or traditionnal CNC.
In this case, download the scad file, change the value of "ouput_type" to "DXF" and then export it to dxf format.