It looks like you all are on the path to completion.
Just another simple solution, if it applies.
I know the OP specifically asked for a Keyboard Maestro solution but I've been using Hazel and Automator, with this conversion, for a few years. I was actually searching in my KM Editor because I thought I had a macro for it (I was wrong).
I'm sure you can replace the Hazel rule, with Keyboard Maestro, to call the Automator workflow but here is what the two activities look like:
Hazel Rule:
Automator workflow:
Very quick and simple.
I'm sure your solutions will work fine so disregard this if your way is satisfactory.
KC