How to Install and Use
Locate the Commands Folder:

Windows: C:\Program Files\MoI 4.0\commands (or your version).

Mac: Right-click the MoI app icon > Show Package Contents > drive_c > moi > commands.

Save the File: Place MakeArch.js into this folder.

Run the Command:

Open MoI3D.

Select 2 Faces (e.g., the top faces of two separate pillars or cylinders).

Press Tab, type MakeArch, and hit Enter.

How it Works
The Logic: The script acts as a "Bridge" tool. It grabs the loop of edges from Face A and Face B and tells MoI to calculate a Blend surface between them.

The Shape: Because it uses Blend (and not Loft), the connection will maintain tangency (smoothness) with the source faces, creating a curved "Arch" look rather than a straight pipe.

Troubleshooting
Twisting: If the arch looks twisted, the automatic alignment of the edge start points might be off. This is common in automated scripts. In that case, use the manual Blend command to adjust the "Seam" points.

Flatness: If the faces are perfectly coplanar (flat on the same ground), the blend might try to stay flat. Angle your faces slightly towards each other for a better arch.

... Moi3D Custom Scripts setup ...

I selected this video because it provides a clear visual guide on how to set up custom scripts and hotkeys in MoI3D, which is necessary to install the plugin code provided above.