This extension for Inkscape creates a locked layer with registration marks typically used by Silhouette CAMEO plotter devices.
- Go to Extensions -> Render -> Silhouette Regmarks -> Classic...
- Edit position and size of registration marks to your liking
- Apply
The registration marks can be used by the "Send to Silhouette" extension at https://github.com/fablabnbg/inkscape-silhouette