Remixed it to provide a magnet, and a slip-fit for changing bits

I printed a few of the original http://www.thingiverse.com/thing:137367 handles for hex bits I use all the time.
They are a very tight fit and will probably never come out.

But I wanted at least one handle to be a universal holder.

So, I edited a cavity in the bottom of the hex to hold a small neodymium magnet, 3x6 mm.
I have a whole bunch of these, hacked from a child's toy magnet construction set. Different sets have different diameter magnets, and some thicker and thinner.

The fit had to be a little sloppy so the bits can slip in and out. Surprisingly, scaling up to 105% was not enough. Perhaps that's because I test-printed only one handle at a time, and the print was really hot and soft, and a lot of shrinkage was the result.
(Maybe it's this brand new spool of filament.. i dunno.)

Anyway, I jumped to 110% (X and Y) and after cooling it is a nice loose fit, similar to a commercial holder.

I "paused" the print to insert the magnet, smooshing a couple strands of soft plastic over it, and that worked OK, but the 3x6 magnet is small enough to drop through the hex-hole and might also be secured with super glue or 5-min epoxy.

Who knows what will happen to the fit if you print more than one. All I know is small, thick parts like these cool to different dimensions when more or fewer are printed on this Afinia. Re-scaling up or down might be needed.

Afinia default setting.. (medium speed, 0.3mm Z, medium fill.)