Summary

This model is a holder designed for Olfa KB-5 blades and can accommodate 4mm blades.
The blades are safely secured by a mandrel system and a blade cover protect it during storage.
The to pieces of the mandrel should be screwed together a few times before first use to remove the burs. The cover is designed to fit very tightly for security reasons. A cutter or a small file should be used after printing to adjust the inner diameter until it fits perfectly into the grove present in the cap.

Edit: I have recently made another holder for 4mm blades. It requires a M3 screw and a melt insert but holds the blade a bit more strongly. You may want to take a look: https://www.thingiverse.com/thing:6805463