I wanted to create a scope that approximately follows the same visual style as the kid's nerf gun. It features crosshairs and is printable without supports. A brim is recommended though, for better bed adhesion.
The fit should be pretty snug, but it has a springy "tongue" that should prevent any rattle. I've included a small test piece you can print to make sure it will fit the rail. Scale the model up a bit in your slicer if it seems too tight.