When creating a custom part that is walkable by crew you might wanna specify where the player may build doors.
You can specify this in the according rules file of your modded part.
The allowed door locations are specified as a list of [x,y] called AllowedDoorLocations.
Following is an example of how the coordinate system works. Coordinates are counted with x to the right and y downwards from the top left cell of your unrotated part.
In source code the Locations would look like this: