The best way to cut an insert opening in a router table is by guiding your router with a flush trim bit against a template made for the insert plate. I make mine from shop scraps and auto body filler. Here’s how: Rip 3"-wide pieces of MDF or other scrap into strips that are about 6" longer than each of the sides of your insert plate. Your template material can be 1/8" to 1/4" or so thicker than the plate and still work fine. Wax the corner areas of your plate to keep glue from sticking to it, and place it on a flat, waxed paper surface. Stagger the strips around the plate so each strip extends past the next. Epoxy the strips together, making sure that all four strips fit tightly around the plate.
Now mix up some quick-set two-part auto body filler and pack it into the corners of the template up to the insert plate to fill the gaps. When the filler dries, carefully tap the plate free and shave away any extra filler from the corner curves. You now have an inside template that matches your router plate’s shape perfectly. The longer “ears” of the template make clamping easier, too.