The following is from bobj3 and I used it successfully to make 2 acrylic router bases. It does not refer to a router insert but the idea is the same. Hope it will help.
To drill my insert I first experiment with pieces of 1/8” plywood which I cut them the same size as my insert. Even after a few trials, one of the mounting holes was still out by about 1/32”. It’s not an easy job.
This is the way I make them (Make a Router Sub Base by bobj3)
I do like to make them 8" x 8" square.
Some of the plastic comes with paper on it if not put some masking tape on it.
Next draw a FINE line from corner to corner ,then draw a line around the corners about 1" from the out side edges.
When the line goes across the X line, draw a small cir.around it, then chuck up a 1/16" drill bit in the drill press ,drill the center hole mark and then the out side holes on all 4 corners,,,
Once you have that done chuck up a 5/16" drill bit (8mm) and drill out the holes on the corners, then chuck up a counter sink bit and just touch the holes you just drilled on both side of the new plate..to remove the small bur..
Then remove the base plate off the router, this will become the template to drill the mounting holes,,,,
This is the hard part because it needs to done just right and on dead center.
Make a disk that just fits the center hole in the plate,,,use a hole saw, for this job, because it will put in a 1/4" hole in the center of the disk, use some 1/4" MDF ,hardboard,plastic,etc...cut the disk out just a little bit bigger than the center hole in the base plate..now with care sand the disk so it can be press into the hole, sand a litle bit at a time, I made a jig to do this job but you can do it by hand but with care.
Once you have the disk made, take the new base plate to the drill press and pop in the 1/16" drill bit, then clamp the new base plate down to the drill press using the 1/16" hole to line up the clamp job,once you have the plate in place and then chuck up a 1/4" drill bit and drill out the center hole, drill out the center hole to 1/4",, then put the disk you made and the old base plate on top of the new plate.
Push the drill bit down into the two holes, this will line all the parts up then with some masking tape,tape the plate down all the way around the old plate...now unclamp the plate and with a 4mm drill drill out the mouinting holes, then put the 1/4" bit back in the chuck and reline the plate and clamp it down again,,,once you have it clamped down remove the old base plate and the disk you made...then chuck up a 1 3/8" bit or 1 3/4" bit..
This come down to what size of guides you want to use.
Once you have the bit in place drill down about 1/8" deep in to the plastic, then chuck up the 1 3/16" bit or the 1 1 /2" bit again this comes dow to what size you want to use,,, now dril out the center hole to the scrap block you have under the new base plate on the drill press.
Once you have the hole drilled out unclamp the base plae and remove the masking tape and the old base plate and chuck up a counter sink bit and counter sink the mounting holes so you mount the new base plate in place.
This is the long way but the safe way, once you do it a time or two you will find the short cuts, but for the 1st.time you need to do it this way to get the hang of it... .
For The bearing you can remove some off the router bits many router bits use the 5/16" (8mm) ID type.. ( 1/2",5/8",3/4" OD the norm) hope this helps a bit