Hello to everyone!
I'm brand new here as you can see from my post count and I hope a long post won't put anyone off.
I am just finishing a home made router table myself RustyMcc, and decided to make my own inserts. It sounds like this might not be something in which you are too interested, but I thought I'd throw it out there for consideration. I built the table from plans at the link below. I found the online format a bit hard to work with so I converted it to a MS Word document and then to a pdf which I attached if that would be of any help to you.
I have the Fast Joint System from MLCS Woodworking and if you don't have your bit centered in the guide within a couple thousandths of an inch your joints turn out loose or too tight. If they're loose, you start over, If they're tight, you spend hours hand tweaking them to get a nice fit. When I decided to build this table, one of the primary things I wanted was a very precise way of centering my guides on the router spindle.
I also have a couple of those old Ryobi RE600 workhorses and they are heavy! They do have a nice soft-start, electronic speed control, and at 3 HP will turn any bit I have including some 3-1/2" diameter panel-raisers. Yesterday I mounted one of the routers in the table as I had decided the best way to guarantee the inserts would be precisely centered was to use the plunge feature on the RE600 to run a 3-1/2" diameter bit up through the table top to end up with a hole I knew was spot on the center of the router. A 1/8" rabbeting bit made a nice 1/4" deep ledge to hold my insert plates as you can see in the first photo.
Today I used a Jasper Tools Perfect Circle Guide with one of my DeWALT DW618 routers and a 1/4" straight bit to cut some 3-3/4" diameter red oak disks from 1/4" thick stock. I cut seven disks and all of them fit absolutely perfectly (one has a small ... uh ... mistake
on the edge but still works just fine). I'm not going to sand them on the edges at all as the fit is so good I'm afraid of ruining it (a few of the disks in the photos still need a little clean up)
The last step is to put the various size holes I want in the disks. I'll use five of the disks for various bit sizes and use the last two for 1-3/16" holes and rabbets for standard guides. To put all these holes in, I'll put a disk in the hole and clamp some backing wood over it and probably use some double-sided tape to make sure the disk doesn't rotate in the hole. Then I'll mount a 1" bit in the router and plunge it up through the disk (and into the backing a bit). From there, I can get any size I want (including holes with rabbets) by alternating a rabbeting and flush bit to gradually increase the hole size.
So far, the plan has worked out as I hoped. I'll have to see if I get the kind of accuracy in the oak disks I need for the Fast Joint System guide bushing.