Hook: Wall Small (Green Square)The Factory Welding Droid decoration fits into a small green square wall hook.
Hook: Wall Medium (Blue Square)The Factory Welding Droid decoration fits into a blue medium square wall hook.
How to Get a Factory Welding Droid Decoration
Star Fortress Decorations
Star Fortresses are a type of solo or group content similar to flashpoints. You can not access Star Fortresses until you have completed Chapter 9 of Knights of the Fallen Empire (level 60+). The enemies in the Star Fortress have a chance of dropping bunch of different Star Fortress and Skytrooper Decorations, including the Factory Welding Droid decoration.
You can usually also find most Star Fortress decorations on the GTN, sold by other players who won them while doing a Star Fortress.