NOTE: First make sure you go into Fortnite's settings and you turn on 'Reset Building Choice'
Below are how the Macros should be set, you can add them to any key or button on the mouse that you would like. This is caused because of the mechanics of the game. The only down side is that you must be selected on your weapons to go directly to a build, if you are already building the Macros must be pressed twice to exit building and then go back directly to the piece needed. I was able to try out some Macros after this new feature was added and now we can set a Macro per building piece.
