I wanted the ability to change bits frequently without eventual loosening due to wear on the plastic - a magnet seemed like the best approach. I have added a cavity that allows for a circular neodymium magnet (5mm diameter, 3mm thickness) to be included, secured with a drop of superglue.
I have included the OpenScad file I wrote to add the cavity to 4volt's original .stl - it is fairly simple to change the values to accommodate different sized circular magnets. All the script does is remove a cylinder from the original design, so you will need to download 4volt's original to the same location as the OpenScad file for it to work. Details of this are included as comments in the file.
To those unfamiliar with OpenScad, let me know in the comments if you need an .stl file for a particular magnet and I will add them when I get a chance.
Securing the magnet
I stuck the magnet to an Allen key (use anything magnetic, a drill bit works well too), added a drop of superglue to the face of the magnet and used the key to press it into the hole.