Utility Knife Locking

** Just added a "stiff" version. No change to external dimensions, but increases the amount of material holding the blade in place - much stronger. The tradeoff is that there will be more friction in opening and closing the blade.

A couple of changes based off of Talon's Utility Knife.

Slightly longer to incorporate 60mm blades as I found they stuck out a bit.
Catch system is simplified, but now locks in both the open and closed positions.
Blade changes are a bit easier.
This design is in metric.
Changed the angle for the blade carrier as it was a bit off for 60mm blades.

Warning: This is a plastic knife to which you will be adding a very sharp blade. Be careful! I expect the locking mechanism will be good for a while, but it will definitely wear loose over time. So don't trust it and be willing to junk it if it wears out.