Add custom plates to your table
Router-table work goes smoother and more safely when the hole in your insert plate is only slightly larger than the diameter of the bit. You can buy a plate with removable rings to fit different router-bit diameters, which gets you close enough in most situations--or you can make a custom plate to match a bit exactly. Use Baltic birch plywood for the least expensive plate, or choose polycarbonate for a clear plastic plate.
Place the insert plate faceup on a flat surface. Remove the subbase from your router, and adhere it to the plate, face up, with double-faced tape. Be sure it's centered, and oriented so that your router will be convenient to operate once it's mounted under the table. Select a drill bit the same size as the holes in the subbase, and chuck it in your drill press. Using the holes in the subbase as guides, drill matching holes through the insert plate. Remove the subbase, and countersink all the holes.
Now, attach the insert plate to your unplugged router and place it flat on your workbench. Chuck a 1/4" drill bit in the router, and lower it until the bit touches the insert plate. Turn the collet by hand to mark the centerpoint.
Remove the insert plate from the router. Chuck a holesaw or adjustable circle cutter in your drill press to cut a center hole of the diameter needed, as shown.