Create holders like this:
Bit/Socket Holders is a FreeCAD workbench for convenient creation of 3D printable holders for hex bits, sockets and other tools. Custom shapes are supported and may be created using Sketcher. Holes for cylindrical magnets are created by default. They can be equipped with magnets of 2mm diameter and 4mm length to hold sockets and bits in place.
Each holder slot becomes a part for use in the Part workbench.
Custom tool shapes can be created using the Sketcher workbench: Construct a sketch consisting of a single closed loop polygon in the XY-Plane. Add Points for magnet holes. The Any Shape Holder tool button opens a dialog which asks for the sketch name and then creates a part object out of it.