Customizable clip that will fit any rounded rectangle. You will need to know the following measurements of the item you want to clip: width, height, corner radius.

To get the corner radius I just held up and compared various circular things until I found one that matched the same curve as the item I wanted to clip. In my case it ended up being a marker cap. I was able to then measure that with calipers to get the diameter I needed.

The specific use case I had this time was to mount the Ethernet Tag Manager from Wireless Sensor Tags. https://wirelesstag.net/specs.html#manager

Modify using OpenSCAD from http://www.openscad.org. Open the included .scad file, modify the variables at the top, render and export your STL.