This design contains two handles for 4 and 6mm wide blades. The blades are hold very secure by a mandrel mechanism and can be used for precision work. The design uses one melt insert and one 25mm M3 bolt to tighten the mandrel.

The 4mm blades i use are the ones sold for ultrasound knifes and other small cutting tools, but they can also be obtained by snapping off thin 30° (9mm) utility knife blades.
The 6mm blades are common for modeling knifes. The blade protection should be adapted for most blade types.

For this model print orientation matters: I print both the handle and the mandrel straight and the layer lines create a "snapping" effect that keeps the mandrel in position when tightened. To change blade you then need to unscrew the bottom know and push it inside the handle.
If you do not desire this effect you can print the mandrel horizontal. I would also advise to do that if you use a filament with poor layer adhesion as it would improve the part's strength.

Edit: I have added different version of the "mandrel" part to adapt to different melt insert diameters. The 3 sizes are 5, 4.6 and 4.2 mm. It should be adapted for most inserts on the market. 

Edit: I also designed a version of the handle for 4mm screwdriver bits and a clip to hold them. Having the option to clip the handle and have it easily accessible is quite handy so i designed a new version of the 4mm blade handle compatible with those clips.