This is a small foldable keychain designed to hold 1 or 2 sections of 9mm blade from a classic box-cutter (snap-off blades).

I wanted something that could cut open post packages and small ropes and fit in my pocket safely so I made this one in solidworks, took me 7 prototypes before finding "the one".

EDIT : I added a new way to insert the blade, it's way better ! The blade was sometimes getting out of her socket, so I redesigned it, now you insert the blade front behind and it locks into place, plus the blade is now on a corner which is way easier to open packages ! The new file for the inside is : 6_int_better.stl

Features :
There is a little protrusion so you can open it with one handed easily between your index and thumb.
It's small, ~3cmx2cm folded !
You can put it on a keyring.
It's supposed to be a tight fit so it shouldn't open on itself in your pocket (nor the blade should come loose). If it does because your printer doesn't have the same tolerances as mine, try heating the sheath a bit with a lighter (2sec) and press gently on the sides, it should tighten the fit.

Printing :
The first file should be sufficient for most printers, put supports underneath but not inside the model (I've included a screenshot of Cura so you'll have an idea).
You can also try the print-in-place version for added strength on the hinge but it's a trickier print. Finally you can print both pieces separately in whatever orientation you chose depending on your printer settings.
If you have any troubles, the easiest way to do it is to print the 4_ext flat on it's side with internal supports, and the 5_int in same way but with supports only underneath.
I should warn this is made for a 0.4mm nozzle, I don't know if the tolerances are okay for a bigger nozzle. I've included the Solidworks files if you want to tweak it.