I made this knuckle iron inspired foldable knife, it has a really steady grip and is great for opening boxes etc. 

The file is sized for a medium size hand but is easy adjusteble by scaling it up or down. Also i recommend scaling the z height to 10/12mm (Just remember you need longer bolts 20/22mm) I printed this with 100% infill for rigidity but its probably not necessary.

It uses:

 4x M4 18mm bolts 

2x M4 5mm bolts

6x nuts

1x Utility knife blade