Use this template to remove the material necessary to accommodate a manual safety. The XMacro has a thicker frame than the normal 365, so I remixed the template to widen it the necessary amount.
Just print the model (I recommend 4 copies because they will likely get damaged or broken in the process), use the FCU pin to hold the template in place at the rear of the grip, and then use your favorite method to remove the material inside the template hole. I found the best way was to use a tiny drill bit to drill some holes along the perimeter, slice between the holes with an Xacto knife, and then finish up with some small files.
You'll also have to remove material from the frame where the safety goes. I used a wood burner to slowly melt away the material. It kind of looks like shit but it works!
The author remixed this model.
Made thicker to accommodate XMacro's grip frame width