Nerf stock attachment

There are a lot of designs for Nerf blasters' stocks available for print, but most if not all of them rely on a friction fit. I took the design closest to what Hasbro had to offer, [made by fantasticmrdavid](https://www.thingiverse.com/thing:2526152), and added to that the missing catch as well as a few personal touches.
The result is a stock attachment dock, available to be added to your favorite stocks.

Beyond the pieces you print, you will need
- 2 nerf screws (#3-24 x 3/8"). the origina