Make your own. Don't drill a standard countersunk hole, it will be almost impossible to center it properly. Drill the center hole to size first, use a centering cone to hold it in place, centered accurately. Then carefully mark the center of each mounting hole with a fine punch. Be as precise as you can. Then drill a hole a little larger than the threaded portion of your bolt (save the ones from the original base). Next, drill just about half way through with a bit a little larger than than the diameter of a flat headed bolt. That little bit of wiggle room will allow you center the base on the cone. I think you could use some 1/4 inch lexan for this, which will make it easy to mark the locations for drilling.
If you use an opaque material, you will need to take the old base off and do your best to mark a piece of paper or chip board accurately, then use that template to mark the new base. You will want to mark the center very carefully, for example, take a pointed bit or short, half inch drill bit, Put the base on and then lower the bit til it marks the exact center, then with a drill press, drill out the opening size you want, probably with a forstner bit.
This is a painfully detailed description, but since you're using bushings, precision will be extremely important. Even drilling slightly off center will introduce errors if the router turns somewhat or changes direction. With this replacement base, you will want to recenter it with a centering cone everytime you remove, then reattach the base.
The more I do, the less I accomplish.