I remembered today that I never posted the jig I made. I made a prototype first out of scrap wood. It worked, but was unstable, mainly because I rushed it. My solution for the stability was to make it out of PVC pipes. It also made the construction much faster.
I sanded the ends of the horizontal pipe that holds the swing arm so it moves more smoothly. Then I hit it with some WD40... just because. It sticks where you put it, which helps make straight cuts, but moves very easily. It works best if you push or pull the jig across the router bit lengthwise, rather than swing it back and forth.
I attached the plywood at the bottom of the arm with two screws and nuts. I stuck a PVC plug at the ends of the pipes so I could attach the screws(I tried to get pictures, but the didn't come out. Sorry if that explanation is confusing).
The best part of the jig is that I can easily more radii to it by adding more pipes. Right now, it's at 12". I can set it up to do compound radii, but I'll need to make some changes to the swing arm first. It probably won't take more than 5 minutes, though.
Most of the parts only cost a few cents.