[DesertRatTom]

A short answer isn't going to do the trick I fear. Lots of us use saw horses and a slab of ply. If it were me, I'd get a small mortising bit. Like this

The word template points to the straight edge. You'll just set it slightly off one edge of the existing cut. You run the bit so the bearing rolls against the template or straight edge and trim off just enough to make the track fit. One nice thing about mortising bits is that they leave a very flat bottom. A half inch mortising bit is all you need. And in future you can use it to make multiple passes for any width you want.

Hope this helps. I also suggest that you buy a sheet of quarter inch masonite with one factory (straight) edge to use for your straight edge in the future.

You can also make a simple jig that's called an exact fit dado bid. Picture attached. You fit the piece you want to fit in the dado between the "jaws" and tighten the nuts so they don't move. Then using a mortising bit that's smaller than the dado or groove, you can get a perfect fit. You might have to fit a playing card in the jaws between the fitted piece and jaws to allow room for glue.

If you build this extremely handy jig, be sure you make the fixed jaw and the guide absolutely square, exactly 90 degrees. It will make the jig very useful. It is also very handy when cutting dados and grooves. If you use half inch for the various parts, it will be more useful.

 

[DesertRatTom]

One more thing. To set depth, I like this little gem. A router height gauge. About $12. https://www.amazon.com/Trend-GAUGE-...ds=Router+Height+Gauge&qid=1634526123&sr=8-15

 

[DesertRatTom]

With all due respect, I suggest you stick with your router. Your old saw may or may not be tuned up sufficiently well to make a perfect cut, and you'll be lining up and making the cut with the piece upside down. A 1/16th trim with a mortising bit will be very easy to set up. And I assume you'll be using screws to hold the track down, so perfection isn't required.

 

[DesertRatTom]

That looks like a neat little device, but I wonder what the distance between the legs that are placed on the router is. It looks like it was made for a router table and too big to fit on my little Makita RT0707C.

It's about 3 inches across the open part, about 5 across the full leg span. I don't know how wide your Makita base is.