Reference to a discussion online.
Here's a picture of the PCB holder I designed. It has magnets in the base, to stick to any metallic surface.
The magnets have a surprisingly strong and stable holding force - enough that the system can hold a PCB on the metal bed of a CNC mill without bolts.
This one has washers at the far ends. This PCB slides in from the front and stops at the washers. Reverse one side (one washer in front, one at the rear) and the PCB is locked in place.
The side slots allow me to hold a PCB vertically or at a 45 degree angle. The elongated holes are for mounting on a mill table, and the threaded holes are for some attachments I made.