Is that the inserts are difficult to make, one for each size of bit. You end up making a different plate for each size and then have to change it out each time. That's why I suggested a commericially made plate here. The inserts just twist in and lock. This link has both: http://www.ptreeusa.com/router_list.htm
Their part no. 1075 is temp out of stock, but very reasonable.You could always take a pre-made plate larger than your table opening and cut it down to fit rather than try to adapt the inserts somehow to fit a homemade plate. There have been times I just needed a larger bit for one operation and had to change the insert back and forth. If you are running one size bit for hours at a time, then it's not that big a deal to change out the entire plate, but I personally wouldn't do it that way. These plates are reasonable enough, have the leveling screws built in and the twist lock insert holes and last a lifetime.
I suppose you could, using a combination of different size hole saws, make your own inserts, but it seems like a lot of work, maybe requiring a lathe. So, you're back to a different plate for each size bit, remove the router, unscrew the router, screw on a new plate for that size bit, etc.... JMO bill
Inverting your router in a router table allows for safer and more versatile operations. Our deluxe router table insert is made from high strength plastic. Large 9"x12"x3/8"(1/4" lip) design accommodates heavy routers.Snap out rings allow for openings of 3-1/8", 2-5/8" & 1-1/4" for a closer match to your router bit. The deluxe router table insert includes a shoulder pin for freehand work and magnets to help hold the insert into your router table. Threaded Allen wrench inserts in each corner help you level and easily adjust the router.
Here's their main catalog page:
Woodcraft also has inserts: