itch.io

I think the simplest method would be to use the Building Tool to paint an area of interior, and then use the Terrain Brush to paint hatched interior around the interior area. You can also accomplish the same thing with patterns & walls, but it'd be more laborious.

I used the Terrain Brush. Background (must be selected as the first color) is Light Grey, then I "paint" the hatchings with ! Hatch Light Grey. 

Have fun