Need some help with a skil plunge router

Hello!

I'm kinda new to woodworking and in my current project I need to add a 5 Inch groove into a narrow board that is only 1 inch wide.

I find it hard to do with the round baseplate that comes with the skil router. I can't seem to find a square version like you often see with trim routes..

What's the right approach here?

Thanks!