That's one of the jigs on my to do list Jim. I tried to make one from the edge guides that came with my 2 big Hitachis but one has fixed rods and they weren't long enough to use the part from the other one. I had to do a job just like the one you show and as careful as I could possibly be with the single edge guide I had a few very small strays off a straight line.
Owners of DIY routers generally don't have the luxury of being able to buy a second edge guide. Many DIY router manufacturers use the same simple guide shown in my photo and having two of them wouldn't help. I made my own second edge guide (also shown in the photo) and use it when cutting mortices with a hand-held plunge router.
It will work just as well for the application you describe.
PS. The cut you show is a groove: a U-shaped cut with the grain. A dado is a U-shaped cut across the grain. Why two different names for what is essentially the same cut. When my grandfather cut a groove he did it with a wooden plane. When he cut a dado he did it with a tenon saw and chisel. Two totally different approaches and hence two different names. The names have lived on.
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