I lost my CH Hansen magnetic stud finder so I made this.

I used a total of 4 20x10x3mm magnets and inserted them into the print with a pause in the gcode.

This stud finder also features a window so that you can mark a center line.

EDIT:  I used it today to install some cabinets and I like this much better than the CH Hansen finder.  The center line marking window was very useful!