To create a circular aperture at the end of the pipe, we will nest 2 surface objects; a rectangular obscuration and a circular (flat) surface made or air. We will list the rectangular obscuration first. Insert an Object # 7 of type Rectangle (surface object) with following parameters.

Ref Object: -3 (referenced to Object #4)
Z position: 50
Material: Absorb
X Half Width: 5
Y Half Width: 5
All other parameters: Default

And Object #8 of type standard (flat circular hole over the object #7)

Ref Object: -1
Material: Blank (leave empty)
Max Aperture: 2.5
All other parameters: Default

If you update the layout you will get the following message

The reason for the geometry error, in this particular case, is because we have violated one the surface nesting rules. One of the rules in chapter 12 says:

“Surface objects may not share boundaries with volume objects unless the surface object is reflective (Mirror) or absorbing (Absorb), or unless the volume object is listed after the surface object; in which case the volume defines the properties of the common boundary”

In this case, object #8 which is made or air (not Absorb or Mirror) is sharing a boundary with object #4. We can move the aperture (object #7 and #8) 1um away from the pipe by making the Z position parameter of object #7 50.001 mm rather than 50 mm.

In the layout above,  the rays outside the circular aperture are blocked as expected.