Hey Simon,
I am currently using Fusion Studio 9.0.1 for macOS, but I’ve also used the tool in the Lite version. I’m not sure about the earlier versions of Fusion, but in this build, you can find the tool by following either of the below steps:
Click on the “Tools” drop-down at top of screen -> Move cursor down to “Resolve Connect” -> Select “External Matte Saver”
OR
Down in the “Flow” window, right-click in the empty space -> highlight “Add Tool” -> then highlight “Resolve Connect” -> then select “External Matte Saver”
-Dylan