Drip channel for cutting boards..HELP!!

I have made several cutting boards this year for Christmas presents. I would really like to start adding a drip channel / groove to them. I dont think I have ever made the same size board twice. This is due to the fact that I been using scraps from other projects. I understand that a jig would be easiest for this task. But since all boards are different sizes, I dont believe a jig would work.

Any suggestions on how to achieve my goals? I really dont want free hand the router like I have read some people do.

I was thinking maybe an adjustable jig, but not sure how to make one of those. Is there any videos or how to documents on this sort of thing? Thanks