If you are freehanding with a router, you can make a u-shaped jig that
is the width of your router base +18mm. Then you use any smaller bit
that is at least 1/2 the width of the final slot (so 9mm min) and do
two passes, one against each side of the jig.
18mm + router
Tip, just use a piece of the material and the router as a spacer while
creating the jig.
Or, in my situation I'd use my RAS and shim a dado blade to the right