Update 3/30 - I have added two versions of this that are in a printer friendly configuration. Type A - the latest version of the original design, and Type B, a new design based on a diopter sight. Let me know if you make one!
I got a new nerf gun at the office and it is woefully lacking in the sight department.
Derived from http://www.thingiverse.com/thing:16091
This thing was made with Tinkercad. Edit it online https://tinkercad.com/things/7cTurDoNKkc
Warning - the original design required some filing to fit on my Raven rail. Type A and Type B represent my latest work. I will update when I print and test them. See the Tinkercad link to see my WIP.